6. Cave of the Swallows (Mexico)
This open air pit cave in Aquismón has a free-fall drop of more than 1092ft; it is the 11th deepest pit in the world and the 2nd deepest in Mexico. Flocks of birds circle the opening of the cave mornings and evenings, occasionally pulling in their wings and free-falling in the depths of the pit (this is how the cave got its name).
Fun Fact: A building like the New York Chrysler Building could easily fit in this cave.