Deep below the seafloor, locked in a layer of 100 million-year-old rock, a lifeform quietly lives. It's not quite Godzilla, nor a long-lost Megalodon, but it does go to show how life on Earth can dwell under the most extreme and bizarre circumstances.

Scientists have recently discovered that communities of microbes living beneath the seafloor are able to survive in rock sediments for over 100 million years with desperately little nutrients. After being coaxed under the right conditions in a lab, the ancient microbes are even able to snap out of their “hibernation” to metabolize and multiply once again.

Reported in the journal Nature Communications today, the researchers got their hands on these microbes by gathering sediment samples from 75 meters (246 feet) below the seafloor in the South Pacific Ocean nearly 5,700 meters (18,700 feet) below sea level. As the researchers discovered, the microbial life was capable of being revived through finely tuned techniques in a laboratory. Incubated with isotope-labeled carbon and nitrogen laced nutrients, within 10 weeks the isotopes showed up in the microbes, demonstrating they were in a metabolically active state, even capable of feeding and dividing.

“These are the oldest microbes revived from a marine environment,” Steven D'Hondt, study author and Professor of Oceanography at the University of Rhode Island, told IFLScience.

“Even after 100 million years of starvation, some microbes can grow, reproduce, and engage in a wide variety of metabolic activities when they're returned to the surface world.”

The microbe communities became trapped beneath the seafloor long ago after being buried by layers of sediment made up of “marine snow,” debris, dust, and other particles. This layer of sediment studied was deposited over a period from 13 to 101.5 million years ago. If the sediment is formed under the right circumstances, oxygen is still just about able to penetration to these depths, but little else can migrate, suggesting the microbial communities have stayed put for all these years. While the layer does contain oxygen, it has very limited amounts of organic material, such as carbon, and is an unbelievably harsh environment for life. 

In the incubated lab conditions, some of the microbes responded rapidly, increasing in number by more than four orders of magnitude over the 68 days of incubation. Even in the oldest 101.5-million-year-old sediment, they observed the microbes uptaking the isotopes and increasing in cell numbers.

Most of the microbes appear to be aerobic bacteria, meaning they are bacteria that need oxygen to survive and grow. Given the scarcity of nutrients that far down, it’s likely these microbes have slowed down their "body clocks" to live an extremely sluggish life, complete with a slow metabolism and very slow evolutionary speed. 

“We believe the community has remained there for 100 million years, with an unknown number of generations. Since the calculated energy flux for subseafloor sedimentary microbes is barely sufficient for molecular repair, the number of generations could be inconceivably low,” explained D'Hondt.

It was once assumed that life could only survive just a few meters beneath the seabed, namely near continental edges where lots of organic matter can be found. However, as this study affirms, researchers are now showing that life beneath the seafloor is much more diverse and fascinating than previously realized. In a separate study published in March 2020, scientists even discovered microbial communities living some 750 meters (2,500 feet) beneath the seabed.