Mayan underworld proves researchers' dream

The ancient Maya once believed that Mexico's jungle sinkholes containing crystalline waters were the gateway to the underworld and the lair of a surly rain god who had to be appeased with human sacrifices

Now, the "cenotes," deep sinkholes in limestone that have pools at the bottom, are yielding scientific discoveries including possible life-saving cancer treatments.

Divers are dipping into the cenotes, which stud the Yucatan peninsula, to explore a vast underground river system.

Hefting air tanks, guidelines and waterproof lamps, they have so far mapped 405 miles of channels that form part of a huge subterranean river delta flowing into the Caribbean sea, and they are only just starting.

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