Hypogene gypsum karst and sinkhole formation at Moncalvo (Asti, Italy).

In the morning of February 15th 2005, during excavation works in an underground gypsum quarry at Moncalvo (Monferrato area, Asti, Northern Italy), a water-bearing fracture was intercepted at level 134 m a.s.l. During the night a large amount of water (approximately 60,000 m3) and mud invaded the quarry tunnels reaching a height of 139 m a.s.l. the morning of the day after. Meanwhile a large sinkhole (20 m wide and 10 m deep) formed on the surface. Hydrogeological surveys were immediately carried out to follow the quickly evolving situation, while speleological and geomorphological fieldwork was made possible only seven months later. Two important caves were discovered both showing clear evidence of a hypogenic origin with sculpted morphologies due to slowly flowing water under pressure. The sinkhole formed by the collapse of one of the main chambers of the biggest of these caves, when buoyant support provided by the water started to decrease due to lowering of the virtual water table. The recharge of this karst system is from below and only very minor quantities of infiltration water come from the above lying surface, as has also been confirmed by hydrochemical analysis. This hypogene karst is completely invisible at the surface and develops entirely underground showing no relation whatsoever with the surface. Its presence is therefore extremely difficult to reveal and such types of karst can thus make up extremely dangerous situations. This is the first example of hypogene gypsum cave related to ascending waters in Italy.