Measurements of permeability of unconsolidated material representative of subsoil types and hard ground at the DOC thermal area were taken using a PL-300 soil permeameter and a portable air permeameter (Supplementary Material 2, 3 and Supplementary Figure S2; Umwelt-Geräte-Technik 2012; Heap et al., 2017; Mayer et al., 2017; Montanaro et al., 2017; Rott et al., 2019). For some of the investigated sites, the permeability of soils or hard ground could not be measured because of the increased pore humidity and/or temperature. A total of 83 soil strength measurements and 53 soil permeability measurements were collected from 25 subsurface soil profiles (Figures 8‒10 and Table 1). Moreover, 197 measures of permeability were taken from hard grounds (Supplementary Table S1). Cylinders of known volume were used to collect samples for laboratory investigations and were weighed to determine bulk density. This technique allowed the preservation of sample texture and water content. Soil strength was determined by a pocket penetrometer (Zimbone et al., 1996) and a torvain (Farquhar et al., 2001), giving unconfined compressive and undrained shear strengths, respectively (Supplementary Figure S2).

All measured petrophysical and mechanical properties were well within the range of the applied instruments. Field methods are often not comparable in terms of the standard error of measurement with well-constrained laboratory methods (Heap et al., 2017); however, they are internally consistent.

Soil samples, sinters, sulphur-cemented crusts, and pumices were further characterised in the laboratory to determine their petrophysical properties (Supplementary Table S2). Selected sinter and crust samples were cored perpendicular to layering, if present, and selected pumice samples were cored parallel to the long axis of the elongated vesicles in the pumice. Samples were dried in an oven at 65°C for at least 24 h until fully dry. Weighing of samples before and post drying allowed determination of their water content. The dry powder density of all samples was obtained by a helium pycnometer (Ultrapyc 1200e^®, Quantachrome). Knowing the volume of the sampling cylinder, as well as the water content and dry density, allowed the determination of the porosity of the samples, which is a key parameter controlling their mechanical behaviour (Pola et al., 2014; Heap et al., 2015).

Domain 1 occupies most of the eastern sectors of the DOC thermal area, and the surficial expressions are aligned along a NE–SW axis (Figures 2, 3 and Supplementary Figure S3). In the central and northeastern portions of the domain (sites 1 and 2), there are abundant natural collapse structures with and without springs and pools at their base. Springs and pools are mostly milky to turbid in colour, with mild to intense bubbling and steaming activity. Collapse pits can be isolated with a 10–30 m maximum diameter or form coalesced structures up to 45 m wide (Figures 4A–C). From NE to SW, the depth of these structures decreases from ∼10 to ∼6 m. Other typical features surrounding the collapse structures and often-occupying excavated areas include sulphur-cemented, steaming and heated grounds with spotted fumaroles (Figures 4B, D). Unstable grounds, consisting of altered, reworked, or in situ material, are normally found in areas between collapse structures. In the southwest portions of the domain (sites 3 and 4), the presence of springs and pools of different compositions characterises depressed areas produced by mining activity (Figures 4E–G). Site 3 is one of the most spectacular geothermal manifestations, informally known as “Middle Earth.” Here, a variety of bubbling milky-to-green waters and blackish mud pools, sulphur-rich deposits, fumaroles, and intensively degassing pools are found within a 50×30-m-wide area (Figure 4E). Site 4, in the south-western part of domain 1 (Figures 4F, G), contains a range of water springs mixed with muddy pools, within a rectangular area.

Domain 2 is located in the central sector of the DOC thermal area (Figure 2; Figure 3; Figure 5A and Supplementary Figure S4) and includes five main sites: sites 5‒7 are located within a ∼180×110-m-large, ∼E–W directed, depressed area dominated by mixed water springs and pools, known as the Lagoon; site 8 corresponds to a ∼N–S elongated, ∼100×60-m depression showing a mixture of water springs and abundant mud pools; and site 9 consists of a series of ∼10×10 m–∼80×70 m large depressions containing a mixture of water springs. All depressions are located within partially or extensively excavated areas related to the mining phase.

Springs and pools in site 5 form almost continuous coalesced structures, ∼100 m long and directed NE–SW, and are mostly milky in colour, showing mild to intense bubbling and steaming activity (Figure 5A). Microbial mats and stromatolites are present at several localities around these hot pools (Figure 5B). Muddy and laminated sinter deposits, with localised hot water springs and stromatolites (Figures 5C, D), dominate site 6. The springs at site 7 include isolated pools 3–30 m wide with turbid waters showing mild bubbling (Figure 5E). The surface outflow area of all springs and pools, marked by a strandline of pumice pebbles, is characterised by the presence of various sinter deposits.

The depression at site 8 is filled with a variety of bubbling milky waters on its western side and bubbling mud pools on its eastern and north sides (Figure 5F). Sulphur-cemented ground can be found in the northern portion, while most of the pools are surrounded by unstable ground and locally by collapsing structures. A zone between the two large depressions is characterised by the presence of a patchy sulphur-cemented and cold ground (Supplementary Figure S4).

The area around site 9 includes a variety of natural and mining-related features, with emergent water springs and/or mud pools, as well as overimposed natural collapsed structures (Figures 5G, H, and Supplementary Figure S4). In particular, the largest depression in site 9 contains mostly milky-coloured water, with a couple of turbid water springs, all showing mild to intense bubbling (Figure 5G). The other depressions show mostly turbid to muddy waters, with mild to intense bubbling activity accompanied by weak degassing (Figure 5H). Unstable ground characterised the rim and inside the walls of these excavated areas, while heated grounds and sulphur cementation had developed on tread tracks and were locally present on the rim and surrounding areas.

Domain 3 occupies the northern sector of the DOC thermal area (Figures 2, 3 and Supplementary Figure S5) and includes site 10, characterised by scattered depressions and collapse pits, varying in size from ∼10×10 m to ∼20×40 m; site 11, comprising two shallow excavated sites from ∼35×40 m to ∼40×60 m large; site 12 is a ∼200-m-long and ∼10-m-wide depression, W–E oriented. Many of the features overlap or develop within mined areas.

In site 10, degassing depressions and collapsed structures surrounded by unstable ground occur with or without bubbling pools (Figure 6A). Site 11 is mostly dominated by sulphur-cemented ground developed on tread tracks, with localised fumaroles and steaming and heated ground (Figure 6B). Site 12 is one of the hottest features in the whole DOC thermal area and is dominated by highly altered, friable, and unstable ground with collapsing rims. The fissure is located at a geological contact—likely a crater rim—between the Taupo Pumice deposit and a bedrock made of Oruanui ignimbrite and Parariki breccia deposits (Figure 6C). At the west end of the fissure, there are excavated areas with unstable grounds and a ∼10 m large collapsed pit, as well as fumaroles and steaming and heated ground (Supplementary Figure S5).

North of the DOC thermal area, within one excavated area at site 13, we found dominant sulphur-cemented and heated grounds surrounding the unstable and degassing areas (Supplementary Figure S6A). Adjacent to this, there is a ∼50×60 m large area with unstable ground and collapsed pits 5–10 m wide, as well as a small excavated zone with degassing pits (Figure 3A). In the northern end of the DOC thermal area, at site 14, a ∼6×60 m large section is characterised by small fumaroles, degassing, and heated, unstable ground (Supplementary Figure S6B). Along the western lakeshore at sites 15 and 16, respectively, we found patches of sulphur-bearing pumice clasts and slightly heated ground, as well as water springs showing weak bubbling (Supplementary Figure S6C). At site 17, siliceous stromatolitic sinters are formed in acid–sulphate–chloride spring outflows (Supplementary Figure S6D; see also Schinteie et al., 2007). Along the Parariki Stream, several other spots are characterised by active degassing with mild to intense bubbling in the water (Figure 2, and Supplementary Figure S6E).

In the NE sector outside the DOC thermal area, within the excavated site 18, there is a large fumarole (at Sulphur Cliff) that is altering a substrate of Oruanui ignimbrite and breccia deposits (Supplementary Figure S6F). In the same sector, there is a series of ∼30×40 m–∼70×90 m large and ∼20-m-deep collapsed structures (sites 19–22) all containing milky waters showing intense bubbling and accompanied by vigorous steaming (Supplementary Figures S6G, H).

Soil layers were categorised as undisturbed vs. reworked and unaltered and altered, as well as depending on their granulometry and average petrophysical and mechanical properties (Figures 7‒10; Table 1). The first group includes soil layers within the in situ Taupo Pumice deposits (T) and generally shows no obvious reworking or mechanical disturbances to the original textures, though their degree of chemical alteration is variable. The main layer types are as follows:

Unaltered or weakly altered Taupo Pumice fine-grained layers (T1). These layers show a light grey to light brown, silt to sand-sized matrix, supporting granule- to boulder-sized pumice (Figure 7A). Layers T1 can vary from matrix to clast-dominated, show a low permeability (4.3×10^–15 to 1.7×10^–14 m²) and low porosity (11%–19%), are hard in consistency (3.7–5.5 kg/cm²), and have a high shear strength (3.3–5.5 kg/cm²).

Moderately altered Taupo Pumice fine-grained layers (T2). In this layer, the primary clasts, matrix, and original colour are recognisable. The matrix is light to brown orange and consists of silt to sand-sized materials, supporting granule- to boulder-sized pumice clasts (Figure 7B). Pumice-rich parts are generally altered, but their texture is still recognisable. Layers T2 have low permeability (1.1×10^–15 to 7.1×10^–14 m²), low-to-medium porosity (18%–33%), is very stiff to hard in consistency (3.5–5.5 kg/cm²), and have a low to very high shear strength (1.5–7.4 kg/cm²).

Strongly altered Taupo Pumice fine-grained layers (T3). Such layers have a white to grey matrix dominated by silt and clay-sized particles, with a minor sand component. Portions rich in pumiceous granule- to cobble-sized clasts show stronger alteration and are much weaker than matrix-rich variants. Pumices may be altered to the point where their texture becomes entirely unrecognisable (Figure 7C). Layers T3 show a low to high matrix permeability (1.1×10^–15 to 1.4×10^–13 m²), medium-to-high porosity (22%–69%), a highly variable consistency ranging from very soft to hard (0.2–5.5 kg/cm²), and have a low to very high shear strength (0.6–4.7 kg/cm²). It is to be noted that the very soft to firm layers (σ<1 kg/cm²) have a double value of shear strength (up to 1.5 kg/cm²).

Weak to moderately sulphur-altered Taupo Pumice coarse-grained layers (T4). These layers are dark grey, pebble- and cobble-rich soils with abundant centimetre-sized sulphur nodules and sulphur crystals <0.5 mm in size (Figure 7D). Such layers generally show a clast-supported texture and develop within coarse layers of the Taupo Pumice deposits in warm areas proximal to degassing sites, where measurable layers T4 show high permeability (2.9×10^–13 m²), low porosity (18%), are stiff to hard in consistency (1.2–5.3 kg/cm²), and have a low to medium shear strength (1.3–4.2 kg/cm²).

Highly sulphur-altered Taupo Pumice coarse-grained layers (T5). These layers are yellow-stained and rich in sulphur mineralisation. The original lithology is often disrupted, though a coarse-grained texture could be recognised, and sulphur is distributed throughout the layer as crystals or nodules less than a few centimetres in size (Figure 7E). Layer T5 occurs nearby (<1 m) to active degassing features, shows low to high permeability (3.2×10^–15 to 8.7×10^–13 m²), low porosity (18%), is firm to hard in consistency (1–5 kg/cm²), and has a low to medium shear strength (1.9–5.8 kg/cm²).

The second group of soil layers is typical of areas that have been excavated for mining (E) and includes mostly reworked materials. These layers may be similar in granulometry and component to the Taupo Pumice type, but the original deposit texture and geometry have been disrupted, and the material has been reworked and/or compacted. This layer group comprises

Sand to very fine gravel dark grey layers (E1). These layers generally are dominated by sandy material and contain <10% pumice with sizes <2 cm (Figure 7F). They can only be found in the excavated area on the eastern side of the DOC thermal area (location of s1‒s4 and e1 at site 1; Supplementary Figure S1A). Layers E1 show a very low-to-low permeability (<5.6×10^–16 to 2.4×10^–14 m²), a medium porosity (32%–47%), a varying consistency from very stiff to hard (2.2–5.5 kg/cm²), and a low-to-medium shear strength (2.2–3.4 kg/cm²).

Silty grey to white layers (E2). Such layers are dominated by a silty matrix and a minor sandy component, with <15% of pumice granule (<3 cm) and sporadic pebble-sized clasts (up to ∼6 cm; Figure 7G). Layers E2 are similar in appearance to T3, but show a lower permeability (4×10^–15 to 3.7×10^–14 m²), a lower porosity (6%–58%), a harder consistency (3–5.5 kg/cm²), and a higher shear strength (2.6–4.8 kg/cm²).

Sandy dark to light pink and brown layers (E3). These layers are very heterogeneous, with significant horizontal variation in texture and physical properties. Pumice content is <30%, with clasts commonly being in the granule to pebble size range (up to ∼5 cm). Brown to dark brown soil lenses and large sulphur-mineralised pumiceous clasts are common (Figure 7H). In the E3 layers, only the properties of more consistent layers were measurable, while very friable lenses were too hot and highly degassing for measuring any reliable value. The measured lenses show low-to-moderate permeability (4.5×10^–15 to 2.7×10^–13 m²), low-to-high porosity (11%–65%), very stiff to hard consistency (2.6–7.5 kg/cm²), and low-to-very high shear strength (2.6–7.5 kg/cm²).

The third group includes layers made of clay-rich reworked and highly altered material (C), which typically lies between the undisturbed Taupo Pumice and reworked material via sharp contacts. Such layers are found in warm areas and in proximity to degassing features. This group includes the following:

Dark red clayey layers (C1). Fine matrix and embedded pumices are generally altered to clay (Figure 7I). The pumice content can vary from absent to up to 30%, with sizes typically in the range of granules to pebbles (<5 cm). These layers show low permeability (4.3×10^–15 to 1.7×10^–14 m²), medium porosity (34%–41%), very soft to stiff consistency (0.2–1.1 kg/cm²), and low shear strength (0.9–2.5 kg/cm²). Similar to T3, C1 material has a shear resistance higher than its normal strength.

Variegated clayey layers (C2). This type of soil layer includes a clay-rich matrix exhibiting orange, purple, and red colours within irregular, streaked patterns (Figure 7J). Altered pumices are a major component (up to 50%), with mainly granule- to pebble-sized clasts (up to 8 cm in diameter). These layers show low-to-medium permeability (2.7×10^–14 to 4.4×10^–13 m²), low-to-medium porosity (17%–44%), variable consistency ranging from very soft to stiff (0.1–1.5 kg/cm²), and low shear strength (0.9–3.3 kg/cm²). C2 also shows a higher shear resistance than its normal strength.

The fourth group is characterised by variegated silty and sandy layers crossed by roots and shows clear signs of oxidation (O). Minor pumices (<5%) of fine pebble to coarse sand size (<1 cm) occur in the soil matrix (Figure 7K). Layer O is present near or at the ground surface in apparently undisturbed areas, with no geothermal manifestations. This layer shows low-to-high permeability (5.7×10^–15 to 1.9×10^–13 m²), low porosity (16%–22%), stiff to hard consistency (1.5–4.2 kg/cm²), and low-to-high shear strength (1.5–6.7 kg/cm²).

The fifth group consists of layers rich in clay commonly found in excavated and depressed areas with mud pools (M). These layers can be laminated and show alternation of beds of clay and very fine sand, with a high water content (Figure 7L). They show a very low permeability (<5.6×10^–16 to 2.8×10^–15 m²), high porosity (50%–59%), and are stiff to hard in consistency (1–5.2 kg/cm²), with low to very high shear strength (2.8–7.1 kg/cm²). As for other clay/silt-rich layers, the M layer shows a high shear resistance.

The sixth group is characterised by a light brown brecciated layer (Br). This layer develops over a breccia deposit found in the excavated area at site 13, with granule- to cobble-sized tuffs and lithics embedded in a sandy matrix that appear generally indurated by alteration. At a representative measured spot, layer Br shows very low permeability (9.4×10^–16 m²) and low porosity (21%), with a very stiff consistency (3 kg/cm²) and low shear strength (2.9 kg/cm²).

In the central and northeast excavated areas of domain 1, subsoils are mostly developed in the Taupo Pumice deposits (s1‒s7 and e1 and e2 in Figure 8 and Table 1). Soil layers of the first group can be found with different alteration states (T1–T5), with more altered layers closer to or in correspondence with degassing spots and fumaroles. The layers filling the excavated depression and covering the surrounding undisturbed ground consist of variably altered and reworked materials from the second group (E1–E3). The contact layers of the third group (C1 and C2) generally separate these two types of layers. Often, sulphur-cemented grounds develop on top of the reworked material filling the depression. The temperature in layers below such cemented grounds ranges between 26°C and 38°C within the first 15–20 cm and increases to 66°C–87°C at a depth >50 cm (s1‒s7). Above the ground and in the proximity of degassing spots (e1 and e2), temperatures can vary between 31°C and 80°C. Southwest of domain 1, layers of Taupo Pumice covered by modern soil layers of the fourth group (O) are predominant (e3‒e5 in Figure 8 and Table 1). Generally, their alteration state increases in the proximity of degassing features (e.g., fumaroles), as shown by exposed profile e5a–e5c, where dominant T2 gradually passes into T3‒T5. Here, the temperature of the measured layers within the first 30 cm increases to 55°C and 93°C (e5a‒e5c).

In domain 2, soils within the depressed areas are dominated by sinter formation and mud deposits (e.g., M layers; s8, e7, and e8 in Figure 8 and Table 1). The surrounding rims are instead characterised by unaltered to moderately altered Taupo Pumice layers. For instance, layers T1–T3, covered by modern soil layer (O), can be found on the rim of the site 8 depression, at locations e8s and e8n. Soil layer temperatures are generally low and range between 15°C and 24°C within 45–130 cm from the ground level.

In domain 3, subsoils show a similar sequence to that observed in the excavated sites along domain 1, but with altered layers being more abundant. For instance, at exposed profiles e9‒e14, the highly altered T4 is often found below T3, which is in turn covered by C1 and C2 or by modern O layers (Figure 9; Table 1). Layer temperature ranges between 12°C and 44°C within 180 cm, while it can increase to 89–97°C in the first 30–40 cm below the ground. Close to fumaroles (e.g., at e14), the temperature above the ground reaches 56°C and 74°C within the first 60 cm, whereas in the presence of sulphur-rich layers (e.g., at e9), the temperature can decrease from >90 to <50°C (Figure 9; Table 1).

In areas with the cold sulphur-cemented ground, north of the DOC thermal area (e.g., s9 and s10 at site 13), subsoils below the sulphur-cemented crusts comprise excavated material (E1‒E3) overlaying Taupo Pumice (T) and/or breccia deposit (Br) layers. The temperature of the layers quickly increases to 64–94°C below the crusts but is normally <40°C within the low-permeable, firm breccia material (Figure 9; Table 1).

Partial measurements possible around collapsed structures of domains 1 and 3 (Figures 4A‒D; Figure 6) showed that the top and subsoils were mostly moderately to highly altered Taupo Pumice such as T3 and T5.

Pumices are widely distributed throughout the field, and especially those with granule to boulder size represent a dominant component of many recognised soil layers. In addition to being found as reworked pumices (or pumiceous material) in subsoil layers and hard grounds, they are more often located in situ within the Taupo Pumice deposit. We measured permeability, porosity, and density of unaltered and altered (sulphur-bearing) pumices along an exposed transect (e3, e4, and e5) southwest of domain 1 (Figure 8 and Supplementary Tables S1, S2). Unaltered to weakly altered pumices are very permeable, with values ranging between 1.2×10^–13 and 1×10^–11 m², whereas their moderately to highly altered counterpart shows permeability between 5.3×10^–15 and 6.5×10^–13 m². In terms of porosity, unaltered pumice shows a very similar range (57.5%–64.1%) compared to the sulphur-bearing clasts (60.6%–61%), whereas the density of the former is slightly higher (0.9–1.2 g/cm³) than that of the latter (0.8–0.9 g/cm³). It is to be noted that the measured porosity and density of sulphur-cemented pumice from pumiceous mounds (s1 and s5 samples in Supplementary Table S2) are less porous (22.4%–47%) and can be denser (up to 1.4 g/cm³).

Three main types of sulphur-cemented ground were recognised in the field based on their texture, structures, and physical properties (Figure 11 and Supplementary Figures S7, S8 and Supplementary Tables S1, S2). Generally, the sulphur-cemented grounds develop on top of either in situ Taupo Pumice deposits or over mixed reworked materials (clay–sand or granule–pebble dominated) related to the mining activities. The first type is flat cemented grounds consisting of hard layers >5–25 cm thick, tabular in shape, distributed over large areas or found in small patches, with surficial patinas and degassing cracks. Surficial patinas can be very fine-grained or show a coarser grain texture (Figure 11A and Supplementary Figures S7A, B). The second type is crusted hummocks, which are mound-shaped structures characterised by a surficial fine- to coarse-grained patina, the presence of cracks, and several small degassing vents. Native sulphur occurs typically as yellow encrustations at these fumarolic vents and along cracks (Figure 11B and Supplementary Figures S7C–E). The third type includes cemented pumiceous mounds and tread tracks. These are abundant around excavated areas with the heated and degassing ground where the precipitation of sulphur encrusts both reworked materials and tracks left by mining machinery (Figure 11D and Supplementary Figures S7F–H). In areas of intense degassing, pumiceous mounds can grow by sulphur precipitation and form patina and cracks similar to hummocky grounds. Wrinkle-like features and fractures are very common and lie parallel to the main cracks, showing a spreading of soils occurring around the central cracks. Overall, it appears that sulphur precipitation is more abundant in coarse-grained levels, independent of the ground type.

The permeability of flat cemented grounds measured in spots without the surficial patina varies between 4×10⁻¹⁵ and 8×10⁻¹² m², with lower values associated with the presence of compact silty or sulphur-rich levels (Figure 11A and Supplementary Table S1). In both heated ground and cold areas, degassing pathways or sulphur-bearing fractures linked to surficial cracks are observed in the soil layers below the encrusted grounds (Figures 11A, B). Crusted hummocks have permeability ranging between 1.5×10⁻¹⁶ and 1.3×10⁻¹² m², depending on the presence of silty vs. coarse-grained levels within their structure. Degassing pathways cross the soil layers below crusted hummocks and are connected to surficial cracks or vents (Figures 11C–E). In the pumiceous mounds, the permeability is highly variable depending on the dominant component and ranges from 8×10⁻¹⁵ to 8×10⁻¹³ m² for variably cemented clay–sand portions (Figure 11F); from 1.7×10⁻¹⁵ to 7.4×10⁻¹³ m² for sulphur-bearing pumiceous pebbles and cobbles; and from 2.2 to 3.5×10^−12 m2 for sulphur-free pumices (see site 2 values in Supplementary Table S1). In all the sulphur-cemented types of ground, the presence of surficial patinas can strongly reduce permeability, as shown by measured values ranging between 2.5×10⁻¹⁵ and 5×10⁻¹³ m² (Supplementary Table S1).

The porosity and density of selected sulphur-cemented samples from flat, hummocky, and pumiceous mounds show a wide range of values, reflecting the inner structure of the samples, the degree of cementation, and the dominant grain sizes. The measured samples are representative of warmer and highly degassing sites close to site 1 and of colder crusts in mildly heated grounds surrounding site 13. In warmer areas, the crusted material has a variable porosity (13.5%–30.9%) and density (2.1–2.4 g/cm³), with the more porous sample showing a texture of porous course-grained clasts cemented by coarse sulphur grains. In colder areas, cemented crusts show a porosity of 17.2%–19.3% and a density of 2.1 g/cm³, mirroring a fine-grained texture of the cement and more widespread sulphur precipitation (Supplementary Figure S8 and Supplementary Table S2).

At the Lagoon, stromatolites and silica sinters are the dominant ground type. In the eastern part of the Lagoon, at site 5, stromatolitic deposits form bands 1–2 m wide that lie parallel to the shoreline (Figures 12A–C). They form two to three zones, including 1) spicular to small diameter (<5 mm) columnar stromatolites with increasing height away from the pools; 2) upward expanding blade-shaped stromatolites, with blades perpendicular to the shoreline and forming a distinct ridge that is higher than the columnar stromatolites; and 3) scattered columnar stromatolites (up to 1 cm high), clusters of bulbous stromatolites, or smooth, flat sinter surfaces (Figures 12A, B). The inner structure of stromatolites is laminated (Figure 12C). The surficial permeability of stromatolites, where measurable on smooth surfaces, ranges between 4.2×10⁻¹⁵ and 1.4×10⁻¹⁴ m² (Supplementary Figure S9A and Supplementary Table S2).

In the western and central portions (sites 6 and 7), silica sinters are made of alternating layers of green, yellow, and grey muds with different degrees of cementation (Figure 5D, 12d‒g and Supplementary Figures S9B–D). Such deposits were found to be composed of kaolinite, elemental sulphur, alunite, and amorphous silica and commonly contain many diatoms according to Krupp and Seward (1987). A deposit of relict sinters >1-m-thick outcrops on a side of the excavated area at site 9 is buried beneath reworked materials. Such outcrops and scattered blocks around the site indicate a wider distribution of the sinter deposits than at present, before mining activity. The upper portions of sinters are richer in granular material more typical of outflow deposits (Figure 12H), likely indicating a lowering of spring level in time at this site. The field-measured surficial permeability ranges between 8.4×10⁻¹⁵ and 1.5×10⁻¹⁴ m². Porosities and permeability of some of the most representative samples measured in the laboratory, instead, are in the range of 65%–78% and 1×10⁻¹⁴ to 7×10⁻¹² m², respectively (Supplementary Figures S9A–D).