The tomb of Seti I located in the Valley of the Kings is often considered as the longest, deepest and most impressive of all tombs. The tomb is decorated with highly refined bas-reliefs and colorful paintings. This decorative style set a precedent for other tombs, who followed it in full or partway. The tomb has several chambers and all of them except two contain well preserved reliefs.(source)
18 Derinkuyu Underground City, TurkeyImage Source: travelandleisure
The underground city was built to shelter 20,000 people together with their livestock’s and food supplies. The multi-level underground city was probably constructed by the Phrygians, an Indo-European people somewhere in 8th-7th BC. Â Although Nevsehir province in Turkey has several underground cities, this is one of the largest excavated city. The 18 levels consists of homes, wine cellars, oil press, workshops, ventilator shafts, access to fresh water and chapel for prayer.(source)
The Củ Chi tunnels – the 75-mile long complex tunnel is a tourist attraction and visitors are encouraged to crawl around in the safe passages. But this interconnected underground tunnel played a big role during the Tết offensive in 1968 (Vietnam war). The tunnels located in Củ Chi District of Ho Chi Minh City served as a base of operation for the Viet Cong, as hiding post during combat and also was used as a supply route.(source)
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20Â Nevada National Security Site, Las Vegasimage Source: reviewjournal
The Nevada National Security Site established on January 11,1951 covers approximately 1360 square miles of the mountainous and desert terrain of Nye county, Nevada. The site established by Harry Truman was used for testing of nuclear devices. The majority of tests—828 of the 928 total nuclear tests—were underground. These underground tests liquefied sands and rocks into glass-like bubbles. The test site offers monthly public tours where cameras, binoculars or cell phones are not permitted.(sources)
The catacombs of Paris are underground ossuaries that hold remains (bones) of more than six million Parisians. The catacomb was founded when the city officials had to solve the problem of the overflowing cemetery in 1774. Exhumed bones were transferred from the cemetery to the reinforced tunnels for two years (1786 to 1788). More remains were added in later years. The catacombs have been called the “World’s Largest Grave” due to the number of people buried within. In 2004, police discovered a fully equipped movie theater in one of the caverns. (source)
The Edinburgh Vaults or the South Bridge Vaults were completed in 1788. The series of chambers in the nineteen arches had 120 rooms or vaults designed for commercial purposes. The vaults originally intended to act as a storage room or a workshop for business and it did begin that way but flawed planning led to the quick abandonment of the vaults. It soon turned into a storage place for hiding the murdered bodies and illicit materials. It was also used to rape unsuspecting women.(source)
The barrel vaulted Dom im Berg  lies beneath Schlossberg, the Styrian capital’s Castle Hill. Known as the ‘Cathedral in the Mountain’, the vault is part of the system of tunnels dug out as air raid shelters during WW II. Today, You can use the elevator to go straight to the top or hang out in the club and gyrate to the electronic music.(source)
With an entrance merely located 500 feet from the famed Hagia Sophia, the Basilica Cistern offers a spectacular walk under the 13 feet brick ceiling. The ceiling is supported by a forest of 336 marble columns comprising of Ionic and Corinthian styles. The walk through the columns  gets you closer to two stone Medusa heads (used as a base). The Basilica Cistern is the largest of the ancient cisterns and was built by Byzantine Emperor Justinian I in the sixth century. The cistern could store up to 2,800,000 cu ft of water.(source)
25Â The Cavern Suite, Grand Canyon Caverns, ArizonaImage Source: travelandleisure
The Cavern suite built in 2010 bills itself as  “the oldest, darkest, deepest, quietest, and largest suite room in the world.†The accommodations are built 200 feet below the ground in Grand Canyon and can accommodate six people at a time. The zero humidity is conducive to keeping the insects and other critters away. The walls are 65 million years old.(source)
