The devil worm is the deepest-living animal. It's redefining life at its limits

When a single worm was discovered in the depths of our planet's crust, it began an extraordinarily lucky series of events that transformed our understanding of life at extreme depth.

If you could drill into the ground beneath you and descend into Earth's crust, you'd be uncomfortable pretty quickly. As sunlight vanishes and you sink a little closer towards the planet's molten core, the temperature and pressure mounts, while deep groundwater fills the cracks and holes in the rock. At torrid depths of 0.8 miles down (1.3km), you'd be forgiven for thinking nothing could be alive down here.

But if you're lucky, you might see them: billions upon billions of bacteria, sometimes forming bright orange or white mats – and if you could whisk out a microscope, maybe even a tiny worm using appendages on its mouth to hoist itself along the rock.  

This is Halicephalobus mephisto, the devil worm, on the prowl for microbes. Though miniscule – only as long as the width of several human hairs – it's a whale in this Lilliputian ecosystem. When it dies in the snot-like microbial film growing on the rock,  scientists imagine, its body is consumed by bacteria and other life forms, helping to fuel this strange, Hadean cycle of life.  

Before the 1980s, most biologists didn't think that life existed more than 30cm (1ft) or so below the ground. The detection of deep-dwelling bacteria – and the 2011 discovery of the devil worm – represented a seismic shift in our understanding of the Earth's depths.

The worm's story demonstrates how life, even animal life, finds a way without the Sun's energy or an oxygen-rich atmosphere, radically expanding what scientists thought was possible for living things and sparking new ideas about life's origins and future.

"If you can find a worm down there, what else are we missing?" marvels geomicrobiologist Karen Lloyd of the University of Southern California. "It's just full of possibilities."

Thanks to an extraordinarily lucky series of events, scientists managed to produce offspring from that single devil worm and have been avidly studying its descendants to understand how the species survives in the hot, dark Earth.

Enter the deep biosphere

The first convincing reports of deep-subsurface life emerged in the 1990s. In 1997, for instance, bacteria were found as deep as 2.8km (1.7 miles) deep inside a gas borehole.

In the early 2000s, the worm biologist Gaetan Borgonie, who founded the non-profit research institute Extreme Life Isyensya in Belgium, wondered whether a group of small worms called roundworms, or nematodes, might also exist deep underground.

Borgonie recalls older scientists scoffing at the idea that more complex life might exist in the depths. But Borgonie, knowing how tough the little animals are, felt it was worth exploring. After all, when the space shuttle Columbia catastrophically disintegrated into the atmosphere in 2003, and an on-board experiment with nematodes fell 64km (40 miles) to the ground, the animals survived – and were found happily multiplying.

In 2008, Borgonie teamed up with other experts in extreme life to visit the Beatrix gold mine in South Africa. At 0.8 miles (1.3km) down, the team accessed holes that had been drilled into the mine's walls to tap the roughly 37C (99F) water inside the rock and let it run through filters designed to catch small life forms, Borgonie recalls. After filtering more than 6,000 litres (1,300 gallons) of water, they caught one tiny worm.

At two other South African mines, they filtered even larger amounts of water – more than 12 million litres at one of them – and found a handful of different species of roundworms, along with........

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