This "crystal cathedral", discovered in 1907 following the collapse of its vault, has been open to the public since 1924. Today, visitors standing on a platform can admire the magnificent 52m/171ft high dome which reaches 60m/197ft in its widest part. Water, seeping through abundantly, adds to the stalactites, which are very numerous in places and form four impressive petrifying fountains. "Son et lumière" presentations reveal the concretions and enable visitors to discover all the facets of the chasm during the descent towards the exit.