Cave of the Swallows – Aquismon, Mexico
The world's 6th biggest void, Cave of Swallows is close to the town of Aquismón. Don't think that the visitor will just see a major opening in the ground. Each morning at dawn, the swallows come in groups towards Tamaulipas looking for sustenance and return at dusk. Such a variety of feathered creatures have lived in this hollow, for so long, that the base of the pit is light to the touch. It is their waste that has developed to cover the ground. The nearby Huastec individuals saw it as a passage to the Underworld.