Stalagmites, but not just stalagmites (my emphasis):
> In one chamber, 336 metres from the cave entrance, are enigmatic structures – including a ring 7 metres across – built from stalagmites snapped from the cave floor.
Predating modern humans:
> Natural limestone growths have begun to cover parts of the structure, so by dating these growths a team led by Jacques Jaubert at the University of Bordeaux could work out an approximate age for the stalagmite constructions.
> They are roughly 175,000 years old, which means they easily predate the arrival of modern humans in Europe. They were built at a time when Neanderthals were the only hominins in the region.
