PLAYA DEL CARMEN, QUINTANA ROO, Mexico - Taking in the surroundings more than 30 feet below the earth's surface, I was astounded at the stark, silent beauty of the Mayan underworld.
To my left were what appeared to be giant snow boulders; around the corner, an overturned scale model of a Roman city next to a full-sized, engraved stone column. Gleaming icicles dripped from the ceiling, and - whoa! There's a sculpture of the Madonna holding the baby Jesus.
No way this could be Hell, I thought happily, shining my light through the darkness. The icons are all wrong.
In fact, this place is about as close to heaven as you can get if you're a scuba diver - although it's rumored a similar environment might exist on Mars - but more on that later.
Maggie Martorell, Erika Hernandez, Victor Rosado and I were swimming in an underground river called Dos Ojos (two eyes) on the Caribbean coast of Mexico's Yucatan peninsula known as the Riviera Maya. To get there, we had to climb down a ladder through the narrow opening of a cenote, or sinkhole, and have our heavy scuba gear lowered to us on a rope. It sounds like a lot of effort, but the eye-popping sights in what the Mayas call tzonot, or ''sacred well,'' are worth the trouble.
Just gearing up on the wooden dive platform was a treat. We stood in the middle of a wide, water-filled limestone cave decorated with stalactites and stalagmites - limestone projections that extend from the ceiling and up from the floor. At our feet, the water was so clear that it appeared nearly invisible. We felt its coolness - 72 degrees - through our thick, neoprene wet suits when we got in.
The cenotes of the Riviera Maya, little-known to anyone outside cave-diving circles 10 years ago, attract thousands each year. What keeps them from becoming overrun is their numbers. Of the nearly 500 cenotes registered among the 100-mile coastline between Cancun and Felipe Carrillo Puerto, explorer Sam Meacham said nearly 150 underwater caves and cave systems covering 380 miles have been mapped and surveyed, including the ninth-longest cave on Earth - the 440,000-foot Ox Bel Ha (Three Paths of Water).
What riddles the Yucatan with sinkholes, according to explorer/dive operator Robbie Schmittner, is that in prehistoric times, it was a coral reef. After the last Ice Age 10,000 to 15,000 years ago, water levels dropped, leaving dry limestone caves. Rain, which contains carbonic acid, dissolved the limestone layers - some faster than others. Cave roofs and walls collapsed or eroded, leaving dripstone formations on floors and ceilings in subterranean rivers. The nature-made sculptures are nothing short of spectacular.
Unlike North Florida's network of underwater caves - which are often deeper than 300 feet and subject to the powerful flow of underground springs - the average depth of the Mexican cenotes is about 45 feet and little to no current. Hundreds of feet of caverns can be explored safely without straying out of sight of surface light, or making decompression stops.
Unlike the colorful reefs of nearby Cozumel, the cenotes do not offer much in the way of marine life. At the rims, you'll find a few catfish and cichlids, and back in the darker chambers, there are small, blind fish and crustaceans that have evolved to live off the water chemistry.
Bil Phillips, 51, a Canadian transplant and renowned cenote explorer, says it's possible similar creatures might exist in underwater caves on Mars.
''That's why I should be the one who gets to go to Mars and go diving,'' Phillips said.
The cenotes also are rich archaeological sites, yielding artifacts ranging from thousands-year-old human and animal bones to pottery shards.
Schmittner and Phillips guide divers into the 38-foot-deep Cenote de los Huesos (bones) in Tankah Park to see remains of ancient mastodons, tapir and deer.
''Kind of an underwater museum,'' Schmittner said.
Most cenote explorers appreciate the importance of the tunnels to the economy, culture, and environment of the Riviera Maya. But they fear that others - Mexicans and visitors - do not.
Meacham, 40, a native of Texas who has lived in the region for 12 years, worries that burgeoning development along the Caribbean coast could poison the cenotes that - however porous - make up the foundation of life in the region. He cites statistics to back up his contention:
_Playa del Carmen, center of the Riviera Maya, has seen its population grow 500 percent since 1990. Tourist visits have risen from less than 400,000 annually in 1996 to 2.1 million in 2004.
_Seventy percent of the drinking water for cities in Mexico comes from groundwater, yet many of the large resorts along the Riviera Maya use deep injection wells to dispose of sewage, threatening to contaminate the aquifer.
_Future plans call for a new airport at Tulum, at the southern end of the Riviera Maya, sitting atop the region's largest concentration of underground rivers.
Meacham is organizing a nonprofit educational foundation to get his message to residents, tourists and politicians.
''We have to be extremely careful in our development plans, or we run the risk of killing the goose that laid the golden egg,'' he said. ``If nobody teaches these [people] what's under there, we don't have a chance of saving it.''
If you go
Advanced open-water scuba divers might be qualified to dive in Riviera Maya's cenotes in small, guided groups, although cavern certification is preferred. Guided dives cost about $100 per person for a one-tank dive. Some cenotes are suitable for snorkeling. Here is a partial list of dive operators: