Riviera Maya, Mexico - With their stalactites and stalagmites, and pools of water shining an unearthly blue, Riviera Maya cenotes look like wondrous underground cathedrals. You can swim and snorkel inside many!
Dolmen de Axeitos - 3600 A. Prehistoric megalithic dolmen near the village Axeitos, parish of Oleiros, municipality of Riveira on the Barbanza Peninsula in the estuary of the Ría de Arousa in the Province of A Coruña, Galicia, in Northern Spain
One of the coolest places I've ever been too. Xcaret Eco Theme Park is located on the Riviera Maya, Cancun. Xcaret’s underground rivers are part of a large cave system that forms deep under the surface of the Yucatan peninsula