This article is from the New York Times and describes a system of caves and underground rivers in Mexico's Yucatan Peninsula which were formed when sea level was approximately 300 feet lower than it is today. When the current period of warming began approximately 18,000 years ago, sea level began rising to the point where it is today, flooding the caves.
Note that caves in carbonate rocks, (which these are) are formed by rain water combining with carbon dioxide, forming carbonic acid, which dissolves the carbonate rocks (primarily limestone). This is an important chemical process going on every day, all around the world. How it might figure into the balance of carbon dioxide in the atmosphere, I do not know. Does anyone?
Divers have gone underground and under water to explore these caves and water ways and have found evidence of human habitation. This they can date very accurately and develop a picture of global climate warming since the last ice age. This matches with many other kinds of evidence demonstrating substantial global warming long before mankind began burning oil and coal. Note also the mention of pollution of this fresh water research caused by careless pumping of human sewage. Read the article. It is fascinating.
May 21, 2002
Divers Discover Maya Relics in Caves That Became Rivers
By ANGELA M. H. SCHUSTER
Nearly 100 feet beneath the Yucatán Peninsula in southern Mexico, cave divers are mapping the world's longest underground river. More important, they are unraveling the mysteries of a fragile ecosystem that may be destroyed before it is fully understood.
That the peninsula is rich in human history is attested by the temples and pyramids built by the Maya during the first millennium. Underground runs a common thread that has woven the fabric of life and directed the distribution of human settlement for the past 10,000 years: a complex system of rivers and natural wells whose formation began more than 100 million years ago, when the peninsula lay beneath a shallow sea.
Over a succession of ice ages, sea levels dropped some 300 feet, exposing the limestone platform that makes up the peninsula. Over time, rivulets of carbonic acid (a byproduct of rainwater bonding with carbon dioxide in the atmosphere) carved out the caverns. When sea levels began to rise with the last ice age 18,000 years ago, the once dry caves began to fill with water, a process that continued until about 1,000 years ago. Collectively, these submerged river systems provide all of the peninsula's fresh water.
By far the largest of the submerged river systems is called Ox Bel Ha (pronounced OHSH bel hah; the name is Mayan for "three paths of water). Its labyrinthine passageways, an estimated 200 miles, wind their way underground within a triangle, embraced on the surface by the resort city of Cancún, the late classic Maya coastal trading center of Tulúm and the inland classic Maya site of Cobá.
Since 1998, an international team of divers — Sam Meacham of Austin, Tex.; Bil Phillips of Vancouver, British Columbia; and Stephen Bogaerts, a Londoner — has been documenting Ox Bel Ha armed with surveying equipment, lights, hard hats and gas tanks. Some dives last more than 12 hours, the time necessary to reach Ox Bel Ha's deepest recesses, map them and safely return to the surface.
To date, the team has charted more than 60 miles of submerged caverns and documented 57 cenotes, or natural wells, and three freshwater passageways just offshore that are connected to the Ox Bel Ha system.
"In the 21st century, the world is focusing of dwindling freshwater reserves and the need to conserve them," said Mr. Meacham, 34, director of Cindaq, a nonprofit organization dedicated to the exploration and protection of freshwater resources in the Yucatán. "To find such a pristine supply of fresh water in this world is quite rare indeed."
After transporting thousands of pounds of gear deep into the jungle on horseback, the team sets up camp near entrances to the cave system, many little more than sinkholes a few feet in diameter.
"Cave divers have always operated at the edges of the envelope, adapting existing technologies and inventing new ones to push further and deeper," said Mr. Bogaerts, 37.
Carrying reels of line knotted every 10 feet to serve as measuring tapes, divers map the chambers and collect samples of underwater life — small fish, blind shrimp, algae. Where passages splinter off, directional markers are attached to lines with arrows pointing to the nearest exit, in some cases more than two miles away.
Besides collecting samples, Mr. Meacham said, the team plans to install data collectors in the caves to record tidal fluctuations and other characteristics of the water, including temperature, flow rates and salinity.
The fresh water flows through the cave passages and empties into the Caribbean, Mr. Bogaerts said, while denser, warmer saltwater from the sea enters the river system through deeper passages as well as through the limestone itself, which functions as a porous membrane.
Besides the river system, the peninsula is pocked with cenotes and sinkholes to the west that appear not to be connected to Ox Bel Ha. Exploration of several in the vicinity of Cobá has yielded evidence of early human occupation.
"We have found hearths and human remains dating to a period when the caves were dry, an estimated 8,000 to 9,000 years ago," Mr. Meacham said. "We have also documented deposits of ceramics and human bones from the Maya period."
Cenotes and caves played an important role in Maya religion: they were regarded as portals to the underworld, a potent realm of gods and ancestors. (The word cenote, pronounced suh-NOH-tee, comes from the Maya dzonot, which means sacred well.) The finds will be left in place, to be investigated by archaeologists from Mexico's National Institute of Anthropology and History.
The divers say they have investigated about half the underground river system.
"We believe Ox Bel Ha is connected to two nearby hanging cave systems, each about 12 miles in length," Mr. Meacham said. "If we add these to what we have already explored, the passageways of Ox Bel Ha will stretch some 84 miles. To document the entire system is simply a matter of time and money."
But preserving it will be a more formidable task, given the development plans for the Riviera Maya, the 70-mile stretch of idyllic coastline between Cancún and Tulúm. Already the area has 22,000 hotel rooms for more than 4 million tourists a year. More than 80,000 rooms are to be added by 2015. Mr. Bogaerts notes that the hotels dispose of sewage through deep injection wells, which deposit human waste 70 to 100 feet underground.
Raw sewage treated with fresh water gradually bubbles up through the saltwater layer, polluting the freshwater aquifer miles inland from its origin.
"A minor cave system near the resort of Puerto Aventuras has already been pumped full of sewage," Mr. Bogaerts said. "What we are seeing is an inadvertent contamination of the aquifer."
Working with Mexico's Secretariat for the Environment and Natural Resources, the team has started an education program in hopes of curtailing future contamination. They will return this summer to resume unlocking the secrets of Ox Bel Ha.