sinkholes
10. Cave of Swallows (Aquismon, San Luis Potosi, Mexico) One of a number of “vertical caves” in Mexico and South America, the Cave of Swallows is the largest cave shaft in the world at 303 by 135 square meters wide, and 333 meters in depth, enough to comfortably house the Chrysler Building if the ground-floor residents of the Chrysler Building didn’t mind sharing their space with a knee-high, insect-infested layer of bat guano. Despite being carpeted with bat poop, the Cave of Swallows is a popular extreme-sports tourism destination for rappellers, bungee jumpers, base-jump parachutists and even hot-air ...
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