We at IFLScience love a cave. The deeper, bluer, and more terrifying the better. We are also always keen to celebrate the weirdest wonders planet Earth has to offer, and as geological oddballs go, the Trabuc Caves in southern France take the cake.

Situated in Mialet, France, the caves are the largest network of underground passages in the Cévennes. They were first investigated in 1823 and have since been explored to almost 10 kilometers (6 miles), although speleologists – people who study caves – believe them to stretch two to three times farther than that, according to Atlas Obscura.

As well as a rich history of human use – the caves have acted as a refuge for various groups since prehistoric times, including the Camisards during the Reformation and, later, “Trabucaïres”, or highwaymen, from whom they get their name – the caves are renowned for a strange and unexplained phenomenon known as the “100,000 Soldiers”.

When exploration really kicked off in 1945, speleologists were met with what appeared to be thousands of tiny soldiers standing to attention (hence the name), but are in fact a unique type of concretion that still to this day have no explanation.

The “soldiers” litter the cave floor, just a few centimeters tall, and as yet have not been found anywhere else in the world.

These mysterious mineral masses form underwater and are made up of 95 percent calcite and 5 percent clay. Each appears to form overlapping discs, perhaps due to varying water levels.

Beyond this, little is known about how they came to be.

Other, more familiar, cave formations, stalagmites and stalactites, form in pairs, on the ground and ceiling, respectively. As water passes over limestone and drips down, these icicle-shaped deposits take shape. The 100,000 Soldiers, however, form without a partner hanging above them, ruling out this means of formation.

Various hypotheses have been posited over the years, ranging from bacteria to electrostatic forces, but none has ever been able to fully explain this oddity.

All right then, Trabuc Caves, keep your secrets.