Part of an ongoing series on the environmental impacts of the Gulf oil spill.

Digging under the patchily oil-splattered white sands of Pensacola Beach, Florida (map), on Thursday, it didn't take long for scientists to strike black gold.

Oil patties and tarballs were discovered as deep as 2 feet (0.6 meter) beneath beaches dirtied by the Gulf of Mexico oil spill—the deepest oil yet found by a team of University of South Florida coastal geologists that's been studying the effects of the oil spill on Gulf beaches since early May. The previous record had been 6 inches (about 15 centimeters) deep, said geologist Ping Wang, the team's leader.

The discoveries suggest that toxic oil lies hidden under even "clean" patches of beaches along the U.S. Gulf Coast—and that oil-spill cleanup crews are only scratching the surface.

Because the buried oil is both harder to clean and slower to break down, it could be a long-lasting threat to beachgoers, both animal and human, experts say.

This "weathered" oil—mainly tarballs and tar mats—began washing ashore around June 23 in Pensacola. (See pictures of Gulf oil atop Pensacola Beach.)

Waves buried much of the oil under new layers of sand, particularly this week, when Hurricane Alex spawned rough seas around the Gulf. (See "Hurricane Alex Pushes 'Worst Oil' Ashore; Cleanup Slowed.")

"This time, we were lucky," said Wang, kneeling by a freshly dug hole striped with ribbons of black tar on Pensacola Beach, which remains open to beachgoers and swimmers, though a health advisory warns visitors away from any obvious oil.

Hurricane Alex's path was hundreds of miles to the east, so the storm's surges had been relatively small along Florida's Gulf coast.

Yet when a bigger and closer storm powers through the Gulf, it could erode beaches and unleash the oil underneath, he said—while at the same time pushing ashore more oil from the Gulf spill, Wang noted. (See "Hurricane Could Push Spilled Gulf Oil Into New Orleans.")

Wang would know: He's studied 2004's Hurricane Ivan and 2005's Hurricane Dennis, both of which totally flooded Pensacola Beach.

Exposed oil, though, may be the least of the worries.

Even without cleanup crews, surface oil disappears fairly quickly as oxygen, sunlight, and oil-eating microbes break it down. Buried oil persists much longer, particularly deep down, where oxygen is in short supply.

What Lies Beneath: Oil Inches Below Beaches

"If [oil's] buried and you have a five-year-old out here next summer building a sand castle and they uncover a layer of tar and oil, that's not going to be good," said Tiffany Roberts, a Ph.D. student working in Wang's lab.

Contact with oil can cause skin irritation, and inhaling evaporated oil particles may cause nausea, headache, and dizziness—ailments already reported by some Gulf oil spill cleanup workers, according to the U.S. Department of Health and Human Services.

That's why Roberts and colleagues are studying how oil is distributed atop the beach—in hopes of discerning a surface pattern that could be used predict the locations of buried oil, she said.

Right now cleanup crews are "dealing with the immediate," she said. Eventually "we've got to figure out what's below."

Like a New Oil Spill With Every Storm

For example, in 1992 crews packed up and left the Valdez site without realizing that vast quantities of oil still sat below the surface—and much of that oil remains underground today, said Boufadel, chair of Temple University's Department of Civil and Environmental Engineering in Ambler, Pennsylvania.

"You can go to a beach and say the beach is clean," he said, "and then a year later a storm hits and you find out that the beach is still polluted."

It's like "getting a new spill" with every storm, Boufadel added.

Unlike the frigid Alaska coastline, however, Florida's beaches are hot and sunny—conditions that may evaporate exposed oil more quickly.

Wildlife Impacts are "Uncharted Territory"

If the spilled oil under Gulf beaches stays buried, it could harm wildlife that nest and feed along the coast, experts say. For instance, some shorebirds eat only small sand-dwelling invertebrates, such as bloodworms, which could be killed by the oil.

"If the oil does get into the surf zone and [poisons] invertebrates that these guys are eating, then the food base is gone," said Riley Hoggard, a resource-management specialist for Gulf Islands National Seashore. "It's going to be tough for some of these shorebirds."

Likewise, four species of Gulf sea turtle hatchlings—which crawl through sand layers to leave their underground nests—could get injured or killed through contact with buried oil on their way out to sea, Hoggard said.

In part to address such threats, the U.S. Fish and Wildlife Service recently began arranging the relocation of some 70,000 turtle eggs from 700 Gulf Coast nests. After the babies hatch in a special facility in a warehouse at eastern Florida's Kennedy Space Center, they will be released on several Atlantic Ocean beaches—on the other side of the state from the Gulf.

The turtles' internal magnetic "maps"—apparently "tuned" to Gulf beaches during incubation—should point the animals back to their native Gulf waters, even with the entire Florida Peninsula in their way, Hoggard said.