A solution to 18th-century cemetery overcrowding, now host over 6 million bones in an eerie underground display.
UNESCO-listed, operated for 700+ years until 2007, now a half-day trip from Krakow featuring salt-carved chambers, St. Kinga's Chapel, and guided tours.
The underground bunker used by Winston Churchill and his war cabinet during World War II, featuring the preserved 1945 Map Room.
Montreal's 20-mile underground network, a global pedestrian hub, hosts shops and entertainment, serving as both winter refuge and a link to the city's transit system.
A natural sinkhole with vibrant blue water and Mayan religious history, reached by descending stone stairs to a 200-foot diameter pool surrounded by lush vines.
Discovered by teens in 1907 during a cricket match, is a stunning underworld attraction featuring dripping stalactites and crystal-clear water.
Over 2,000 years old and the world's largest, offers an underground amusement and wellness park with a Ferris wheel.
A 3rd-century BC cave temple complex in Sri Lanka, houses a 100-foot-tall gilded Buddha, mural paintings, and over 150 statues.
Offers a unique underground experience with illuminated walls from a high population of glowworms.
A 1st-century AD tomb for Nabatean King Aretas IV, features a six-column facade leading to a colossal inner chamber.