Mexico's water experiences run across Gulf, Pacific, and Caribbean coastlines and through a cenote (limestone sinkhole) freshwater system unique in the world. Baja California Peninsula whale watching tours — grey whale nursery encounters in the San Ignacio Lagoon, where calves approach the boats and can be touched, are the most extraordinary cetacean experience available anywhere. Holbox Island bioluminescent kayaking, Yucatán cenote snorkelling and cave diving through the Sac Actun underwater river system (the world's longest), and Cozumel coral drift diving add the Caribbean register. Puerto Vallarta sunset sailing tours, Huatulco bay boat excursions along the Oaxacan coast, and Copper Canyon river rafting in the Sierra Madre complete a water geography of continental variety.