Some 250 million years ago, a part of Ukraine was under a shallow ocean. When the ocean dried up, it left behind a huge deposit of salt.
The Waitomo Glowworm Caves attraction is a cave at Waitomo on the North Island of New Zealand, known for its population of glowworms.
The Superconducting Super Collider (SSC) (also nicknamed the Desertron) was a particle accelerator complex under construction in the vicinity of Waxahachie, Texas.
The Mir mine, also called the Mirny mine, is an open pit diamond mine located in Mirny, Sakha Republic, in the Siberian region of eastern Russia. The mine is >525 meters (1,722 ft) deep (4th in the world) and has a diameter of 1,200 m (3,900 ft), and is one of the largest excavated holes in the world.
Kaymaklı Underground City is contained within the citadel of Kaymaklı in the Central Anatolia Region of Turkey.
The complex contains over five hundred nuclear-hardened concrete military bunkers, partially buried underground and protected by thick berms of earth, to resist a surface blast wave, as well as radioactive fallout.
Özkonak Underground City is an ancient city built on the northern slopes of Mt. Idis about 14 km northeast of Avanos in Turkey.
Kupari was once a hugely popular holiday resort, until it was damaged during the Croatian War of Independence.
Riese is the code name for a construction project of Nazi Germany in 1943–1945, consisting of seven underground structures located in the Owl Mountains and Ksiaz Castle.
Germany’s “treasure chamber” is located in a disused mine near Freiburg. Documents from over a thousand years of history are stored in the underground archive.
Situated in the west of Bosnia on the border with Croatia, Objekat 505, as it was officially known, was the largest underground airport in the Balkans. The primary purpose of the Objekat 505 was to house a long-range radar early warning system, akin to NORAD, as well as to provide a strategic command center for the country’s defense.
Carrie Furnace is a former blast furnace located along the Monongahela River in the Pittsburgh area industrial town of Swissvale, Pennsylvania.
The Derweze Crater, also known as the Door to Hell, is a natural gas field in Derweze (also spelled Darvaze, meaning “gate”), Ahal Province, Turkmenistan. The Door to Hell is noted for its natural gas fire which has been burning continuously since it was lit by Soviet petrochemical scientists in 1971.
The Kelvedon Hatch Secret Nuclear Bunker at Kelvedon Hatch, in the Borough of Brentwood in the English county of Essex, is a large underground bunker maintained during the cold war as a potential regional government headquarters.
Derinkuyu Underground City is an ancient multi-level underground city of the Median Empire in the Derinkuyu district in Nevşehir Province, Turkey.
Pamukkale, meaning “cotton castle” in Turkish, is a natural site in Denizli Province in southwestern Turkey. The city contains hot springs and travertines, terraces of carbonate minerals left by the flowing water.
Metro-2 in Moscow, Russia, is the informal name for a purported secret underground metro system which parallels the public Moscow Metro. The system was supposedly built, or at least started, during the time of Joseph Stalin and was codenamed D-6 by the KGB.
The Post Office Railway, known as Mail Rail since 1987, is a driverless underground railway that was built to move mail between sorting offices.
Hang Son Doong is a solutional cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam. As of 2009 it is the biggest known cave in the world.
Balaklava at one time worked at the base; even family members could not visit the town of Balaklava without a good reason and proper identification. The base remained operational after the collapse of the Soviet Union in 1991 until 1993 when the decommissioning process started.