The largest known vertical shaft in the world, the 1,200-feet-deep (366m) Cave of Swallows is so big you could fit the Eiffel Tower inside – or the Statue of Liberty twice. Located in San Luis Potosi, in central Mexico, it’s home to thousands of birds and in recent years has become popular with base jumpers. Local guides offer tours but you may not be allowed to enter in wet weather, so it's best visited during the dry season.

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