Bacteria live after 250 million years spent under the ground.
The bugs were discovered in salt crystals buried almost 2,000ft below the ground in a Texan radioactive waste dump.
Scientists isolated spores from a rock sample which they managed to revive after placing them in a growth medium of amino acids.
Previously bacterial spores found in a bee preserved in amber in the mid-1990s were thought to be the oldest living organisms. They dated back a mere 25 to 40 million years.
Bacterial spores are survival structures produced by a few microbe families that allow bugs to remain in a state of suspended animation for long periods of time.
They have a thick protective protein coat, and their cytoplasm - the cell 'jelly' - is partially dehydrated and mineralised.
One intriguing implication of the new find is that it makes the idea of living things travelling large distances through space more plausible.
The suggestion is that seeds of life in the form of DNA or dormant microbes may be carried by asteroids or comets, or drift in interstellar clouds, to fall and germinate on suitable planets such as the Earth.
Travelling at the speed of light, the nearest star to the Earth would take 4.2 years to reach and the nearest galaxy 2.2 million years.
But even huge distances like these might be within range for bugs that effectively live forever.
The 250 million-year-old microbes were discovered by Russell Vreeland, from West Chester University, Pennsylvania, and colleagues in October 1998.
They were in a drill sample obtained from a 1,850ft deep air intake shaft at the Waste Isolation Pilot Plant (WIPP) in the Chihuahuan desert of south-east New Mexico.
The WIPP is the world's first underground dump for the permanent disposal of radioactive waste left over from nuclear weapon production. It contains deep disposal chambers dug out from a 2,000ft-thick salt formation that has been stable for more than 200 million years.
Vreeland's team found the bugs in a tiny pocket of brine within one of the salt samples.
The findings were reported yesterday in the journal Nature, which also included an accompanying article by Bristol University geomicrobiologist John Parkes.
He said the implications of the discovery were 'profound,' raising the possibility that bacterial spores may 'effectively be immortal'.
But before the results could be fully accepted they would need to be replicated by other groups and other studies would have to be conducted on different salt deposits.