The Sawmill Sink blue hole lies in a rocky pine forest in the interior of Abaco Island in the northern Bahamas (see map). It was once a dry cave that filled with water as ancient sea levels rose.
Today the sinkhole measures 55 feet (17 meters) across and 110 feet (33.5 meters) deep.
Scuba diver Brian Kakuk, a consultant for the Bahamas museum, discovered the first Sawmill Sink fossil—a 2,500-year-old giant tortoise shell in excellent condition.
Further remains found in the underwater cave range from 1,000 to 4,200 years old.
The most intriguing find to emerge so far has been the remains of an ancient, terrestrial crocodile.
"We believe that this species is almost the same as the Cuban crocodile," Albury said, of which "there are maybe 3,000 of them left" in the wild and none in the Bahamas.
The rare Cuban crocs live primarily in the water but can also roam on land.
Still, Sawmill Sink would have been a somewhat unlikely location for the croc's ancient ancestors, at 2 kilometers [1.2 miles] from the nearest body of marine water or mangroves.
Another mystery concerns why the crocodiles and some of their contemporaries, such as two extinct species of tortoise, vanished from the island—although researchers have their suspicions.
"The climate and environmental conditions back then weren't much different from those of today," said David Steadman, a University of Florida ornithologist and study co-author.
"The big difference is [likely] us. When people got to the island, there was probably nothing easier to hunt than tortoises, so they cooked and ate them," he suggested.
"And they got rid of the crocodiles because it's tough to have kids playing at the edge of the village where there are terrestrial crocodiles running around."
But of everything the sink has yielded so far, plant fossils could turn out to be the most intriguing finds.
By examining leaves, flowers, fruits, and seeds alongside the animal remains preserved with them, scientists can form a much more complete picture of the ancient environment.
For example, the presence of bracken ferns suggests the island once boasted a regenerative, fire-swept landscape of open grasslands.
Fossils of burrowing owls and meadow larks, which live in such areas, add further convincing evidence.
"There are some really incredible plants coming out," Albury said. "Some of them still have chlorophyll [pigment]. When they are brought to the surface, they are still green."
Stephanie Schwabe is director of the nonprofit Rob Palmer Blue Holes Foundation based in Charleston, South Carolina, and was not involved in the Sawmill Sink dives.
While acknowledging the outstanding preservation of the finds at Sawmill Sink, Schwabe suggests that other blue holes could yield more paleontological and archaeological riches.
"We have thousands of different cave systems all over the Bahamas, and the majority of them haven't even been explored yet.
"In almost any inland cave [where I've dived] I've found the remains of some animals. You can't bury anything. If anything is going to be preserved in the Bahamas, you're going to find it in the underwater environment."
For now, though, Sawmill Sink remains one of a kind.
"I've been working in the Bahamas for 25 years and [been] diving in many blue holes," said Thomas Iliffe, an underwater cave expert at Texas A&M University at Galveston.
"Sawmill Sink is a unique site. I've never seen [such an] abundance or preservation of fossils anywhere else in the Bahamas—or anywhere else in the world, for that matter."
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