Mirny Diamond Mine

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.

Project Riese

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.

Željava Air Base

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.

Derweze Crater

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

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.

Pamukkale

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

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.

Hang Son Doong

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 Submarine Base

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.