Running 124 miles (200 kilometers) and 65 feet (20 meters) under the 14th, 15th, and 16th arrondissements in the City of Light, the Paris Catacombs are home to bones from more than 6 million bodies. Why were the catacombs built? The city's overflowing cemeteries caused a crisis in the 18th century that led the government to move the bones to an existing network of abandoned mines and passageways that ran under the city. Book a guided tour to learn more about its macabre history.

The Wieliczka Salt Mine is a UNESCO World Heritage Site and was one of the oldest salt mines in operation (over 700 years) until 2007. Here, you'll see salt-carved chambers, caves and tunnels, and brine lakes on this fascinating trip 442 feet (135 meters) beneath the medieval town of Wieliczka. This landmark is best explored on a guided tour to learn the most about its history and explore the intricate details of the St. Kinga's Chapel, a religious sanctuary carved entirely from salt deposits.

Rated as one of the best things to do in London, the Churchill War Rooms offers a chance to experience the bunker that sheltered then–Prime Minister Winston Churchill and his war cabinet during German bomb raids. This underground historical site, located below the streets in Westminster, was used as the headquarters to strategize the Allied route to victory during World War II. Don't miss the Map Room, which has remained in the same condition since 1945.

Exploring underground cities sounds like something out of a sci-fi adventure book, but the 20-mile (32-kilometer) network of shops, restaurants, and other entertainment attractions underneath the streets of downtown Montreal is one of the largest underground pedestrian networks in the world. More than just a refuge in the city's freezing winter months, this subterranean attraction provides a home for artistic displays, event spaces for festivals and cultural events.

This natural sinkhole, with an open top to the sky, is part of Ik Kil Archeological Park and is also known as the "Sacred Blue Cenote," thanks to the vibrant color of its water and its history as a Mayan religious site. Climb down curving stone stairs to cool off in the subterranean freshwater pool, which is 200 feet (60 meters) in diameter, about 157 feet (48 meters) deep, and surrounded by lush hanging vines.

Discovered by two teenagers while running after a ball during a cricket match in 1907, the Crystal & Fantasy Cave is one of Bermuda's most stunning natural wonders and a popular underworld attraction. This unique underground cavern in Hamilton Parish is home to dripping stalactites and shimmering crystal-clear water.

Dating back over 2,000 years, the Turda Salt Mine (a 4.5-hour drive from Bucharest) is the largest in the world. This offbeat underground destination, located 368 feet (120 meters) below the earth's surface, also functions as a subterranean amusement and wellness park with a Ferris wheel, an actual lake for boating, a bowling alley, and even a halotherapy (salt therapy) spa.

Dating back to the 3rd century BC, The Golden Temple of Dambulla is the largest and best-preserved cave temple complex in Sri Lanka. Five caves were converted into temples and are now a UNESCO World Heritage Site. In addition to visiting this underground wonder, you'll see a 100-foot-tall (30-meter-tall) Buddha gilded in gold at its entrance, mural paintings on the cave walls, and more than 150 statues honoring Buddha, gods, goddesses, and royalty.

Just a little over a 2.5-hour drive from Auckland, you can go underground to see the walls of the Waitomo Glowworm caves beautifully glow with blue-green light generated by the high population of glowworms. Waitomo, a Maori word that translates roughly to "water passing through a hole," is exactly what you'll experience on a tour of Waitomo Caves, thanks to a boat ride that gently glides along the river through the underground complex.