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Swimmers float in the sapphire waters of the Ik-Kil cenote, near the Maya site of Chichén Itzá in Mexico’s Yucatán Peninsula. Cenote means “natural well” in Spanish. Sinkholes occurring at sea level will fill up as high as the water table, creating the famous clear blue pools, used by the Maya royalty for both relaxation and ritual sacrifices.

More sinkholes at: http://www.chilloutpoint.com/featured/9-world-famous-pits-and-sinkholes.html#1567