Tree Rings and Sinkholes Reveal How Abandoned Slate Mines Slowly Fall Apart

Deep beneath the forested hills of the Nízký Jeseník Upland in Czechia, a hidden crisis is unfolding. Decades after the last slate was hauled to the surface, abandoned underground mines are quietly deforming, collapsing, and reshaping the landscape above them. A new study published in Environmental Earth Sciences by Jan Lenart of the University of Ostrava and colleagues examines three remarkable gravity-driven failures in these legacy mines—flexural toppling, a major rockfall, and a collapse-induced sinkhole—and argues that such events cannot be understood in isolation. Instead, the researchers contend, mine collapses are the visible endpoints of cascades of weathering, water flow, and air circulation that link the underground workings to the wider geosystem in which they are embedded.

The hazard is far from trivial. Abandoned underground mines worldwide have produced some of the most dramatic geotechnical failures on record. In Pennsylvania, USA, a continuous discontinuity in a limestone mine triggered the failure of 35 supporting pillars, collapsing the overburden into a 3-hectare subsidence zone and generating an airblast strong enough to propel three miners roughly 15 metres out of the entrance portal. In Tennessee, the successive collapse of 40 pillars over 5.7 hectares produced a seismic event of magnitude 3.1, a destructive airblast, and a sinkhole affecting 3.6 hectares beneath 122 metres of overburden. A 2005 cascade pillar failure in a Chinese gypsum mine killed 37 people and injured 38, destroying 88 residential units; the deadly airblast underground, with a calculated discharge of nearly 187,000 cubic metres, lasted 67 seconds, and the pressure waves reflected repeatedly through the workings, striking miners multiple times.

The consequences can extend far beyond the immediate collapse. Infrasound waves from mine collapses in South Korea were detected more than 200 kilometres away. In China’s Datong coalfield, toxic carbon monoxide migrating through ground fissures after sudden collapses killed four people between 1993 and 2005, required medical treatment for 546 others, and forced the relocation of approximately 9,000 residents. Salt mines fail in their own distinctive fashion: at Solotvyno in Ukraine, uncontrolled water inflow dissolved the salt dome itself, creating suffusion sinkholes 150 to 230 metres in diameter, while a Romanian salt mine collapse after heavy rainfall in 2001 contaminated the Olt River with brine. Even seismic events can be misread—researchers initially attributed a magnitude 2.4 signal at Lo Tacón in Spain to an earthquake, before demonstrating that the rapid collapse of roughly 20,000 cubic metres of rock had generated the signal itself.

Czechia’s slate mining district offers a more intimate scale, but the underlying physics is the same. The Lower Jeseník Upland, part of the Central European Variscides, is built from Lower Carboniferous slates and greywackes deposited as deep-water flysch sediments. Slate mining there is documented from 1776, and expanded dramatically after Emperor Joseph II’s Fire Patent restricted wooden roofing, making fire-resistant slate a commodity. By the Industrial Revolution, more than a hundred quarries and mines supplied the entire Austro-Hungarian Empire. The expulsion of the German-speaking mining community after the Second World War stripped the region of experienced owners, skilled workers, and crucially its mine maps, and underground slate mining finally ceased after 1989. Today, 112 abandoned underground slate mines have been documented in the region, many with unknown extents and no reliable plans.

The first case study, the Kunz Mine, is a six-storey excavation established in 1850 on a steep valley slope, extending roughly 80 metres below the surface—about 50 metres beneath the valley floor and below the groundwater table. Its lower four storeys have been flooded since the Second World War and are accessible only to sport divers. The mine was cut into a complex flexural structure whose steeply dipping slate layers are affected by near-surface creep, and the researchers classify the resulting failure as flexural toppling, a mechanism typical of closely fissile rocks such as slate, in which layers bend toward excavated voids after detachment along bedding planes. Excavation released stress in the rock mass, producing buckling and separation cavities, while recurrent rockfalls released during freeze–thaw cycles along a cubic network of fissures now litter the accessible passages with debris.

What makes the Kunz Mine scientifically exceptional is the team’s pioneering use of dendrogeomorphology—tree-ring analysis—to date the surface expression of an underground failure. The researchers sampled 23 Norway spruce trees showing signs of disturbance, extracting paired increment cores and cross-sections from roots intersecting tension cracks. Cross-dating against a reference chronology from 20 undisturbed trees revealed 44 growth disturbances between 1926 and 2017, including compression wood formation, growth suppressions, root exposure, and an abrasion scar. The analysis identified at least four distinct periods of gravitational activity: 1942, 1973, 1997, and 2008–2009, with exposed roots dating a crack-opening episode to 1970–1973. Notably, the 1997 activity coincided with a year of exceptionally high precipitation in northeastern Czechia, but no comparable rainfall anomalies explained the other event years, suggesting that microclimatic factors rather than precipitation alone drive the instability.

The second case, the Woodboys Mine, captured a rockfall in progress. Sometime in 2022, the toppling of an overhanging wall in a terminal room measuring 28 by 11 by 9 metres released a slab roughly 0.4 metres thick with planar dimensions of 15 by 12 metres—an estimated 72 cubic metres of rock. The layers landed largely intact within 8 metres of the wall base before fragmenting on impact, scattering debris up to 10 metres away. The freshly exposed scarp told the story of the failure’s long preparation: ochre-stained surfaces marked bedding planes where iron oxyhydroxides had developed and the wall remained mechanically intact, while dark grey coatings and flower-like aggregates of gypsum crystals, roughly half a centimetre across, documented a pre-existing loosened cavity. Scanning electron microscopy with energy-dispersive spectroscopy confirmed the crystals as gypsum, most likely produced by the weathering of pyrite in the slate—a chemical clock ticking for years before the collapse.

The third site, the Soví kámen Mine, is one of the largest abandoned slate mines in the region, a multi-storey system spanning about 120 metres of vertical extent. Here, a sinkhole roughly 4 metres deep has developed in a large room, funnelling spoil and collapsed blocks into an unknown void below, while an underground stream that once flowed through the entire mining level now sinks into the debris just metres from the sinkhole margin and actively propagates erosion upstream. The researchers propose several scenarios: a former interconnecting chimney used by miners for gravity drainage may have been progressively destabilized by fluvial erosion, or a major ceiling collapse in the weakened, fault-fractured rock may have breached the rock divide between levels, redirecting the stream into the new connection. Distinct upward airflow from the sinkhole base during the cold season confirms that air and water still circulate freely through the seemingly isolated workings.

This circulation, the authors argue, is the missing ingredient in most collapse analyses. Because mine entrances sit at different altitudes, the systems are microclimatically dynamic: cold air descends and becomes trapped, freezing dripping water into icicles and frost, while warmer air masses interact with humid surfaces to produce condensation, fog, and dripping. At the Devil’s Mouth Shaft, cold air pooling at the base drives frost weathering through microgelivation and ice segregation, and heavy forestry vehicles travel directly over its weathered roof only a few metres above. Slate itself is chemically vulnerable: oxidation of iron sulphides generates sulphuric acid that accelerates alteration, while gypsification produces crystals that exert crystallization pressure within microfractures, progressively delaminating the rock. Hydric expansion and contraction of clay minerals, repeated hydration cycles of gypsum, and post-mining stress release along cleavage planes all conspire to weaken the workings long before any visible failure.

Crucially, the study shows that collapses rarely arrive unannounced. Geotechnical precursors—minor rockfalls, crack opening, acoustic emissions, changes in water inflow, tension cracks, floor heave—may persist for years before major failure, and many are identifiable in the Czech mines today. Yet accurate prediction of delayed sudden collapse remains impossible, and the mine environment is never in equilibrium: like a system repeatedly seeking balance, each collapse redistributes stress and drives further instability in cascading cycles. The researchers conclude that hazard assessment must therefore consider the broader spatiotemporal relationships among landscape elements, overlapping mine levels, and geological structure, and they call for individualized rather than standardized management. With recreational cabins already perched above the collapsing Kunz shaft and construction on undermined ground a persistent temptation, the authors urge that abandoned mine entrances be mapped together with their underground extents—and that loading the fragile rock above them be avoided before the landscape settles the matter on its own terms.

Subject of Research: Gravity-driven collapse mechanisms in abandoned underground slate mines in Czechia

Article Title: Collapses of legacy mines in a spatial and evolutionary context

Article References: Lenart, J., Kašing, M., Tichavský, R., Schuchová, K., & Šilhán, K. (2026). Collapses of legacy mines in a spatial and evolutionary context. Environmental Earth Sciences, 85(15), Article 385. https://doi.org/10.1007/s12665-026-13110-0

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13110-0

Keywords: abandoned mines, slate mining, flexural toppling, rockfall, sinkhole, dendrogeomorphology, weathering, gypsification, pyrite oxidation, geohazards, Czechia, Environmental Earth Sciences