Our planet's murky deep sea sediments are a buzzing hotbed of life, according to a report in Nature magazine.
These bacteria may affect some of the Earth's geological processes
Scientists suggest between 60 to 70% of all bacteria live deep beneath the surface of the Earth, far from the Sun's life-giving rays.
Some of the new bacteria identified are about 16 million years old, surviving 400 metres below the sea bed.
This hostile habitat might be where life first evolved more than 3.8 billion years ago, researchers believe.
"There is evidence that life evolved in the deep sediments," co-author John Parkes, of Cardiff University, UK, told the BBC News website.
"There is clear evidence that life existed more than 3.8 billion years ago. Although, for there to be a big enough biomass for us to detect it in the rocks, it must have been evolving long before that."
But before that time, the surface of the Earth was a brutal place, battered by space rocks and volcanic eruptions.
So, Dr Parkes thinks deep sediments may have been the kindest place for life to begin.
"It might be that life was developing in the sub-surface long before [3.8 billion years] where it was protected from meteorite impacts," he said. "And as soon as the surface of the Earth became more hospitable, the bacteria were able to move up and colonise it."
Far from believing life began in the depths, traditional wisdom dictated it could not exist there at all.
"There was a classic publication in the 1950s that said life stopped a few metres below the sediment surface," said Dr Parkes. "And now we find organisms in excess of 800m deep."
Scientists had good reason to assume life could not live far beneath the Earth's surface. Life needs energy, and there was no obvious source of it.
"The normal view of life on Earth is that the majority of life is on the surface, fuelled by sunlight," Dr Parkes explained. "And you don't expect a large population - even bacteria - to survive away from that source.
"But we are finding that there are a lot of geological sources of energy below the surface. For example, there are a lot of processes that produce hydrogen, which is a good source of energy for bacteria."
Evidence of life in ancient rock sediments was found some time ago but, until now, it was assumed that most of it was long dead.
In the past, scientists have stained bacterial cells so they stood out against the sediment background, but that method cannot differentiate between living and dead cells.
Dr Parkes and his team used a new technique that could identify living cells - and they were surprised to find about 30% of the cells in deep sediment samples are in fact alive.
"We took sample sediments and we stained just the living components in each cell, so we were able to count the living cells," said Dr Parkes. "It reinforces the idea that this large bacterial biosphere is in fact living rather than fossil bacteria."
Some of the cells are imbedded in sediments that are many millions of years old, which means they must be ancient, too.
"These bacteria are growing very slowly in the subsurface," said Dr Parkes. "They could effectively be immortal."
Although these bacteria must seem very remote to us ground-dwelling creatures, we probably feel their influence, the researchers think.
Not only do they influence the balance of greenhouse gases - by both producing and consuming CO2 - they also affect some of the Earth's geological processes, Dr Parkes believes.
"Because they play such a major role in the biochemical processes in the subsurface, clearly they are driving lots of the reactions that produce the chemical steady state on the Earth," he said.
"Possibly, we might not have oil and gas formations without them. So the Earth's biosphere and its geosphere are not separate."