Underneath the small town of Folx-les-Caves in Belgium, are six hectares of manmade caves that can only be reached using a narrow staircase, 15 meter below the ground.

Room after vaulted room, resting on mighty pillars carved out of the tuff stone. It is unknown when exactly they were constructed, and though some suggest they were made in prehistoric times using aurochs horns, it is more likely that they were begun between the late Roman times to the early middle ages. Nonetheless, the size of the ancient mine is quite incredible, and the caves are certainly large enough to get lost within.

Made from tuff, a kind of soft, easy to carve volcanic rock, the tuff, rich in calcium carbonate, was mined to serve as fertilizer. Later the caves harder rock was mined for use in local building projects.

Over time, the caves have served as a refuge and hiding place from various occupants, from possibly as early as the Romans up to the Nazis. Some occupants left sculpted graffiti in the stone, and it can be difficult to tell which era those date from. Among those who hid in the caves was Pierre Colon, a famous robber who has gained a reputation as the Belgian Robin Hood.

The 1965 Flemish youth television series Johan and Alverman (a mix of history and fantasy) was also shot in part in the caves. Today the cave is also used to grow mushrooms.