During the Mesozoic era, the expanse of land from the south central region of Texas to the southern-most extent of Belize into Guatemala was covered by ocean waters. For millions of years, reefs were build by sea corals and animals. These ancient marine deposits gave rise to a thick layer of limestone which was variously uplifted, distorted, and compressed to form the Sierra Madre Oriental, the coastal ranges of mountains, and the coastal plains. The last region to emerge from the ocean was the Yucatan Peninsula. Rain falling on limestone creates a weak acid which over millions of years dissolves the rock along the paths of least resistance giving rise to solution caves. During the most recent ice age, highly decorated caves were formed in the Yucatan Peninsula. These are now water-filled due to the rising of ocean waters when the earth once again warmed, melting the polar ice caps to their present size.
For millions of years, water filtering through this limestone produced vast stores of water reservoirs in the form of underground rivers. Cenotes yield surface access to these subterranean passages. Water pouring forth from the base of the mountains and from natural springs are the nacimientos, or birth places, of these water-filled caves.
These limestone solution caves, nacimientos, and cenotes are the sites of the exploratory efforts of El Proyecto de Buceo Espeleologico México y América Central.