Scientists have revived a 250 million-year-old unit of bacteria found buried beneath the earththe oldest living thing ever brought back to life.

The organism was found in a tiny, fluid-filled bubble inside a salt crystal 1,850 feet underground, about 30 miles east of Carlsbad, N.M., when scientists pulled about 220 pounds of rock salt from the Waste Isolation Pilot Plant, an underground nuclear waste dump.

Fifty-six crystals that showed no signs of contamination were sampled for the presence of bacteria. One crystal the size of a large postage stamp contained the organism. Two other strains of bacteria were found and are being studied.

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If the discovery by Pennsylvania and Texas researchers holds true, it could help biologists calibrate the evolutionary clocka timeline of how species developed over timefor the bacterium and its present-day relatives, said Russell Vreeland, a study author and biologist at Pennsylvania's West Chester University.

The bacteria could open a window onto a prehistoric world that was both dying and being reborn. It would also show the tenacity of life in the toughest conditions.

At the end of the Paleozoic Era, the area around Carlsbad was a vast and barren salt lake. The world was then experiencing its greatest loss of life ever. Up 95 percent of all marine species became extinct. The first known dinosaurs date to about 230 million years ago.

"The end of the Paleozoic was such a curious time and we don't really know what happened," said Renne, who was not involved in the research. "This offers the possibility that we may be able to interrogate some of the organisms that were around."

The findings were published in Thursday's issue of the journal Nature.

The researchers are confident that the germ has been locked away in the crystal all these years. Fossils and radiation tests show that the formation where the sample was found is 250 million years old, they said.

Still, there is the possibility the bacteria somehow seeped into the salt more recently in small drops of water, said Chris McKay, a biologist at NASA's Ames Research Center.

"Unlike amber or rocks or permafrost, salt is not an impermeable material," he said.

The testing of the organism was done inside a containment lab at the Pennsylvania campus. The scientists said they took pains to prevent contamination.

The researchers believe the bacteria survived as a spore and metabolized very little or not at all over the years.

Spores are well-known for their longevity. They have been found in a 118-year-old can of meat, and yeast has been cultured from a 166-year-old bottle of porter ale, R. John Parkes of England's University of Bristol said in a Nature commentary.

In 1995, researchers at California Polytechnic State University reported reviving Bacillus bacteria spores from the gut of a bee stuck in amber. The bee was estimated to be 25 million to 30 million years old.

Since 1960, researchers have reported finding organisms up to 650 million years old in salt, but the findings were met with skepticism because of contamination fears.

In any case, the latest study shows that life can exist inside a salt crystal.

"So the next time you sprinkle salt on your food, think of what else you might be eating," Parkes said.

The bacteria's age easily beats longevity records set by other organisms revived from apparent suspended animation.

"Jurassic Park was neat, but this beats it hands down," said Paul Renne, a geologist at the University of California at Berkeley. "The idea of having a living glimpse of what life looked like 250 million years ago is pretty spectacular."

DNA tests indicate the prehistoric germ is related to present-day Bacillus, a type of bacteria found in soil, water and dust.

"We all feel reasonably comfortable that this particular organism isn't going to attack anything," Vreeland said.

By MATTHEW FORDAHL