Right now, in the hot, dark hell thousands of metres below your feet, there’s a sprawling Lost World teeming with life. It has been there for countless millions of years, yet its very existence has long been dismissed as a scientific impossibility.
No longer. Last week, a leading team of researchers announced there is now “overwhelming evidence” for vast colonies of life-forms deep underground, and called for a global project to find out more about them.
Once dismissed as the fantasy of one maverick researcher, the reality of the so-called Deep Hot Biosphere has profound implications for the existence of life.
And it could revolutionise transform the search for new reserves of gas and oil.
All this seemed unthinkable twenty five years ago this month when Thomas Gold, a controversial scientist at Cornell University, New York, went public with his “crazy” idea that life could exist deep in the rocks of the Earth’s crust.
Back then, biology textbooks insisted that ultimately every organism needs access to sunlight. Everything from plants and plankton to humans gets the energy it needs either directly or otherwise from photosynthesis, which turns light energy into the chemical energy needed by cells.
But in a 1992 research paper in the prestigious Proceedings of the US National Academy of Sciences, Gold challenged this view by arguing that microbes could exist in the cracks and pores of rock far below the Earth’s surface.
In these sunless depths, they would need a radically different source of energy to survive. Gold claimed it could come from oil and other hydrocarbons in the rocks around them.
For the rest of his life, Gold would struggle to convince others to take his idea seriously. Some geologists were so outraged by this “outsider” and his wild theory that they petitioned the US government to have references to it expunged from the nation’s libraries.
But Gold was no stranger to controversy, and had a reputation for radical scientific ideas.
During the 1940s he had been part of a team of astronomers who rejected the idea of the universe beginning with a Big Bang. They argued it had always existed, with matter being constantly created by a special energy field.
Bitterly attacked by some astronomers, this so-called Steady State model was later disproved by the discovery of the heat left over from the Big Bang.
Gold had more success with equally daring ideas in fields ranging from auditory science to the study of the Moon.
By the 1980s Gold had turned his attention to the Earth, and decided to challenge the standard account of the origin of hydrocarbons like gas and oil.
As every schoolchild knows, these are the result of decaying vegetation and organisms being chemically transformed by pressure and heat over millions of years as they become crushed by overlying layers.
But Gold pointed out that hydrocarbons had also been found on other planets devoid of life – suggesting that they may have been present inside the Earth when it was formed 4.5 billion years ago.
The idea that oil, gas and other hydrocarbons might have a non-biological origin was not new. In the 1860s, the French chemist Marcellin Berthelot showed that “organic” compounds could be made using lab equipment.
Gold went further, however, claiming that the hydrocarbons trapped within the Earth were the energy source for vast colonies of bacteria lurking kilometres below the surface.
To back his claim, Gold pointed to reports of living microbes being found in oil samples taken at huge depths.
Critics dismissed this as nothing more than contamination, prompting Gold to put his theory to the test. In the mid-1980s he persuaded the Swedish state energy board to drill for oil where conventional theory said none could exist, and showing it contained live bacteria.
Drilling down over 6km into solid granite in central Sweden, engineers found tonnes of a light petroleum liquid, along with entirely new forms of bacteria.
Sceptics remained unconvinced, and by the time of his death in 2004, it seemed that Gold’s idea of a global subterranean biosphere had died with him.
Yet a major review of the evidence published last week in the Proceedings of the NAS tells a different story. Coinciding with the 25th anniversary of Gold’s original paper to the journal, it shows how scepticism about Gold’s claims is turning into increasing acceptance.
According to a team led by Prof Daniel Colman of Montana State University, Gold’s theory has inspired a new generation of researchers, leading to a host of discoveries about bacteria and the Earth.
It is now known that bacteria do exist at huge depths. In 2006 scientists at Princeton discovered an entire colony of bacteria more than 3,000 metres underground in a South African gold mine.
A few years later, more were found 1.4 km beneath the sea floor in the North Atlantic – and they were using hydrocarbons as a source of energy, exactly as Gold predicted.
Many bacteria are now known to be able to cope with the heat deep within the Earth. Some so-called thermophilic (“heat-loving”) microbes thrive at a scalding hot 122°C. Almost a third of known types of bacteria are also known to have the ability to process hydrogen – which makes little sense for organisms inhabiting the Earth’s surface.
Quite how far down the Deep Hot Biosphere goes is still anyone’s guess. In October 2014 researchers from Yale University found signs of bacterial activity in rocks over 19km below the San Juan Islands near Seattle.
Prof Colman and his colleagues are now calling for a major research programme to explore the mysterious world beneath our feet. They want to see scientists working with oil and gas industry engineers to dig deeper.
The findings are likely to cast new light on the origin of life not only on Earth but on the planets and beyond.
But they may also have more down-to-Earth matters. Last week the UK Engineering and Physical Sciences Research Council (EPSRC) announced a new project to use bacteria in the search for new oil reserves.
The idea is to map the distribution of thermophilic bacteria which might seep from sub-sea oil fields. Trials are currently under way off Canada’s Atlantic coast.
The most important legacy of Gold’s daring hypothesis has much wider implications however: that in choosing a theory, there is – as he put it himself shortly before his death – “no virtue in being timid”.