Japanese scientists say they have revived microbes that were in a dormant state for more than 100 million years.
The tiny organisms had survived in the South Pacific seabed - in sediment that is poor in nutrients, but has enough oxygen to allow them to live.
Microbes are among the earth's simplest organisms, and some can live in extreme environments where more developed life forms cannot survive.
After incubation by the scientists, the microbes began to eat and multiply.
The research was led by the Japan Agency for Marine-Earth Science and Technology and published in the journal Nature Communications.
"When I found them, I was first sceptical whether the findings are from some mistake or a failure in the experiment," lead author Yuki Morono told AFP.
"We now know that there is no age limit for [organisms in the] sub-seafloor biosphere".
Professor and study co-author Steven D'Hondt, from the University of Rhode Island, said the microbes came from the oldest samples taken from the seabed.
"In the oldest sediment we've drilled, with the least amount of food, there are still living organisms, and they can wake up, grow and multiply," he said.
Previous studies have shown how bacteria can survive in harsh places, including around undersea vents that are devoid of oxygen.
Mr Morono said the new finding shows that some of Earth's simplest living structures "do not actually have the concept of lifespan".