What lies below the water of The Bahamas? A veiled world of fossils, blind creatures and scientific riddles. In next month's issue of National Geographic magazine, an international team of cave divers led by anthropologist Kenny Broad of the University of Miami reveals the mysteries hidden from vacationers' view.

The two-month expedition, paid for by the National Geographic Foundation, was merely a small slice of time in a years-long effort to uncover the secrets of this realm, which has been plumbed by researchers for at least three decades.

Only a few miles inland from the Bahamas' sparkling coral reefs, the islands' limestone boasts dozens of submerged caves, "blue holes," some of them hidden in what look like island swimming holes linked to the ocean.

But swimming holes they are not. The inland caves on five islands sport freshwater caps covering heavier saltwater layers, sometimes filled with clouds of poisonous hydrogen sulfide released by salt-eating microbes, acting to preserve whatever falls within. Others contain whirlpools powered by the tides.

Photo: Scientist Kenny Broad fights to the surface in a sinkhole on Abaco. The caves can contain dangerous whirlpools.

Mysteries within: From a cove on Long Island, the cave known as Dean?s Blue Hole plunges more than 600 feet into darkness.

Photos by Wes C. Skiles, National Geographic

"Cave diving is really about knowing your limits," Broad says. "But it provides one of the most amazing experiences in life, and the scientific opportunities are tremendous."

Says cave diver and geologist Patricia Beddows of Northwestern University in Evanston, Ill., who was not part of the expedition: "Each one of these cave diving expeditions, without fail, provides an enormous amount of information. Cave diving is an extraordinarily powerful tool to allow us to get into the heart of karst (cave) systems worldwide."

In submerged caves such as Stargate on Andros island in the Bahamas, the expedition team reports:

• Specialized "chemosynthetic" bacteria that live without oxygen and feast on chemical reactions possible only in the caves.

• Stalactite curtains, or "speleothems," that contain a record of past sea level and climate conditions locked in their structures.

• Fossils of Lucayan tribe members who lived on the islands until the 1500s.

"We've brought scientists from many disciplines together so our results inform each other's work," Broad says. "The initial exploration is just a proof of concept. We still have lots of work ahead."

Coastal regions of the Caribbean, such as Florida, Cuba and the Yucatán, and other regions worldwide contain limestone permeated with caves, Beddows notes, leading to such blue holes. In addition to their scientific value, they often serve as freshwater resources for increasing numbers of people in those regions, which makes investigation of them essential for public health.

Sea level about 20,000 years ago, during the height of an Ice Age, was hundreds of feet lower in the Bahamas. The transition has left its mark on the speleothems and geology of the caves, Broad says, making each one a laboratory for measuring the effects of past changes in climate.

Each cave diving expedition requires years of preparation, and the results will affect scientific conclusions for years afterward.

"It's possible to make a direct comparison between what we do and exploring outer space," Beddows says. Years of training for short investigations in a hostile environment are the hallmarks of cave diving science, which she describes as a "select" discipline, slowly growing in numbers but now increasingly established.

Alongside the danger, the blue holes of the Bahamas have offered little allure for divers who in some cases could instead walk to beaches with access to some of the world's most beautiful coral reefs.

"Why dive into a muddy-looking hole when you can head for the beach?" Broad says. "But (the holes) are really fascinating places once you start looking."

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