7 of the World's Most Impressive Subterranean Structures

Since starting operation in 1905, the El Teniente mine south of Santiago, Chile, has only continued its expansion. Located in the in the Andes mountain range, the world's largest copper mine continues its growth by digging ever deeper to extend its productive life. Overall, the mine has nearly 2,000 miles of underground tunnels and almost 1,000 miles of underground roads.

The discoveries keep coming in the Cappadocia region of Turkey, where teams have repeatedly unearthed massive underground cities, some of which experts say date back as far as 3,000 B.C. The region was already designated a World Heritage site for its more than 200 underground cities and villages, but a 2013 find was deemed the largest yet, found inside a 110-acre zone.

Safety first is the idea behind the creation of Vivos Europa One. Originally carved out of solid bedrock under a 400-foot-tall mountain to house Soviet military equipment, the now privately-owned structure offers 76 acres mixed above and below ground to serve as the "world's largest private underground shelter for long-term protection for families." With nearly 228,000 square feet of secured areas, including 3.1 miles of continuous tunnel chambers, Vivos offers quite the underground protection. If you can afford it.

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It may not have a catchy name, but the world's largest underground flood diversion facility is stunning nonetheless. At 72 feet below the ground about 20 miles from Tokyo, the facility has 59 pillars—each weighing 500 tons—that support the ceiling over this 580-foot-long, 255-foot-wide and 82-foot-long structure that remains open to the public for tours, save, of course, for the few times a year it gets used.

Once the Homestake Gold Mine in Lead, South Dakota, the continent's largest and deepest former gold mine has given way to the Sanford Lab, almost 5,000 feet below ground. And there's room for expansion, as the mine dips 8,000 feet below the surface. Stretching to the equivalent of more than six Empire State buildings, the lab at 4,850 feet deep allows the study of dark matter, shielded from interfering cosmic rays by the mass of earth above it. 

Starting in the 13th century and continuing through to 2007, the Wieliczka Salt Mine outside of Krakow, Poland, saw crews pull salt from the 1,000-foot-deep mine that stretches around 180 miles in length. With such an influential history, the mine was put to use instead of merely closed and is now full of galleries and chambers on the nine floors—with three open to the public 

There are thousands of subway stations the world over, but none may prove more beautiful than the 2012-opened Toledo Metro Art Station in Naples, Italy. One of the deepest in the line, the station is 164 feet down but still spacious at 463,000 square feet of open air. The multi-level station integrates the remains of walls from the Aragonese period in the late 1400s and includes a blue mosaic that grows more intense as visitors descend. The subterranean lobby connects with the popular district above via natural light streaming in via cones in hexagonal patterns.