Mysterious structures found deep inside a French cave are the work of Neanderthal builders who lived in the region more than 100,000 years before modern humans set foot in Europe. The extraordinary constructions are made from nearly 400 stalagmites that have been yanked from the ground and stacked on top of one another to produce rudimentary walls on the damp cave floor. The most prominent formations are two ringed walls, built four layers deep in places, which appear to have been propped up with stalagmites wedged in place as vertical stays. The largest of the walls is nearly seven metres...