10. Sac Actun Cave System
Mexico is known to harbor some of the most remarkable ruins, beaches and forests on the planet, but it is also a land of truly monumental Cuevas submarinas, underwater caves. Underwater cave systems in Mexico include watery labyrinths so large and extensive that they have yet to be fully explored. In one dramatic case, two cave systems were found to be an enourmous single system when a connetion was discovered.