Breathing regular air on Earth, humans can deal with about 8 times sea level pressure before taking harm, which would be at a depth of about 20 km.
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Some cities are so top-secret they can't even be found on a map.Others are padlocked, hidden in the forest, or buried deep underground. From Poland's 800-year-old Wieliczka Salt Mines to America's WWII mystery towns, SPYSCAPE explores some of the world's most intriguing secret cities.
The largest underground city in the United States is the SubTropolis, which is an artificial cave located just above the Missouri River in Kansas City, Missouri. This underground city is 5,100,000 square meters and sits on a 1,100-acre piece of land with limestone deposits dating back to 270 million years ago.
Butte underground city, MontanaThese are well-preserved relics from the early 20th century, when mining was at its height and the population swelled to around 100,000. Space was at a premium so people started using underground areas connected by tunnels. Today, you can usually visit on a subterranean tour.