"The largest manmade sandstone caves in Europe, known as Pekelné Doly, or “Mines of Hell,” lie in a forested area near the Svitavka River. Dug in the 18th century to collect sand for the production of glass for mirrors, the mining operation excavated tunnels a whopping 37,600 square feet in total, supported by sturdy rock columns."


www.atlasobscura.com/places/pekelne-doly-mines-of-hell
www.pekelnedoly.cz/