La Verna cave

La Verna is a show cave in the commune of Sainte-Engrâce in the department of Pyrénées-Atlantiques in France. of mined tunnel leads into the Salle de la Verna, the largest chamber in a show cave in the world. It has a diameter of , a height of , a surface area of and a volume of . A river cascades into the chamber from halfway up the east wall, and sinks into boulders near the base of the chamber.
The chamber was named after the Lyon scouts, La Verna Troop, who helped in the attempted rescue of Marcel Loubens who died following a fall during the 1952 explorations.
In 2003 a standard 4 person hot-air balloon was flown in Salle de la Verna.
Geology.
La Verna is part of the , , and Gouffre des Partages cave system which has eleven known entrances. Explorations still continue in this and in other systems within the extensive Pierre-Saint-Martin karst area where 13 underground rivers and a total of of passages, chambers and shafts have been mapped.
Most of the Gouffre de la Pierre-Saint-Martin is formed by dissolution in Cretaceous limestones, and the main river reaches a base level where it flows over insoluble schists of the Paleozoic basement rocks. The Salle de la Verna has formed where the river flows off the schist onto Devonian limestone. Over time, the river found a route through the soluble limestones, leaving the original downstream river passage (the Gallerie Aranzadi) high and dry. The chamber was formed by a process of solution and collapse, beginning about 200,000 years ago. The unconformity between the Paleozoic and Mesozoic rocks is clearly exposed in the walls of the chamber.
The river flowing through the chamber originated from the infiltration zones on the limestone plateaus, and emerges at springs lower, in the valley of Saint-Engrâce.
Fauna.
In this mineral world lives a unique community of animals, adapted to the dark depths of the karst. They are small invertebrates, blind, and without pigmentation. The two most common species observed in La Verna are the "Aphaenops loubensi" and "Aphaenops cabidochei". In order to survive, these insects need an atmosphere saturated with humidity. After fecundation, the female lays a single egg, out of which a small larva hatches. Contrary to insects on the surface, this larva immediately metamorphoses to an adult, without feeding. The biologist studied these animals in the 60’s. Since the opening of la Verna to visitors, in 2010, a team of researchers of the French national museum of natural history, under the leadership of Professor Arnaud Faille, closely follows their evolution.