The ‘devil worm’ in the depths of the Earth is changing what we know about the limits of life

Deep beneath the Earth's surface, in regions where sunlight does not reach and temperature and pressure increase, it was long assumed that life ceased to exist. However, a microscopic roundworm found in deep mines in South Africa has strongly shaken this assumption.

Known in the scientific world as *Halicephalobus mephisto* and referred to as the “devil worm,” the species was discovered in hot underground water at a depth of approximately 1.3 kilometers. This creature, which is about the thickness of a human hair, is positioned almost like a large predator relative to its size in the deep subterranean ecosystem formed by bacteria and other microorganisms.

Before the 1980s, many biologists thought that life ended a short distance below the surface of the soil. From the 1990s onwards, the discovery of bacteria in deep boreholes and mines began to change this view. However, the ability of a multicellular animal—a nematode—to live under these conditions has become a more striking turning point for deep biosphere research.

UNEXPECTED DISCOVERY IN THE MINES

Gaetan Borgonie, a worm biologist and founder of Extreme Life Isyensya in Belgium, conducted sampling at the Beatrix gold mine in South Africa in 2008 alongside researchers working on extreme life forms. The team passed samples taken from holes reaching water at approximately 37 degrees Celsius within the rock through filters designed to capture microscopic organisms.

After filtering more than 6 thousand liters of water, the researchers found a single small worm. Later, much larger quantities of water were examined in other mines; in some samples, different species of roundworms, invertebrates, fungi, and various microscopic life forms were also detected.

The individual found in the Beatrix mine was incomplete; its tail had been severed during filtration. Despite this, the discovery was of great scientific importance. The new species was named *Halicephalobus mephisto*. The individual was female and parthenogenetic, meaning it could reproduce without mating. Before it died, it laid eight viable eggs.

The lineage from these eggs was later studied in the laboratory of John Bracht, a genome researcher at American University in the USA. When the devil worm was raised under conditions similar to another well-known roundworm species, *Caenorhabditis elegans*, the differences became apparent.

Instead of swimming in water, the devil worm showed a tendency to cling to surfaces. Scientists believe this may help it hold onto rocks and crack surfaces underground. A more remarkable difference emerged in temperature: while this species slows down at room temperature, it thrives and reproduces rapidly at approximately 37 degrees.

LIFE MECHANISM OPERATING AT TEMPERATURE

DNA analyses conducted by Bracht and his team in 2019 showed that the devil worm has an unusually high number of genes associated with heat shock proteins. These proteins provide protection against the damage that high temperatures can cause to cells and proteins.

In studies published in 2024, the *cytochrome oxidase c* molecule, which is important for energy production, was examined. Experiments revealed that this molecule functions effectively in the devil worm at high temperatures, while it deactivates at room temperature, slowing down energy production. This is considered a mechanism that could explain why the worm becomes immobile in cool conditions.

Researchers are investigating whether the species' capacity for asexual reproduction also plays a role in its adaptation to deep subterranean life. This is because the probability of individuals of the same species encountering one another in crack and water systems kilometers deep may be low. In such an environment, being able to reproduce alone can provide a critical advantage for the continuation of the lineage.

How these creatures reached such depths is still not fully understood. Some research suggests that roundworms may have been transported from environments near the surface to the depths via water, and that seismic movements may facilitate this process. In contrast, some bacteria are thought to have existed in the Earth's crust for much longer.

Studies on the deep subterranean biosphere make questions about the limits of life important not only for Earth but also for other planets and moons. If complex organisms can persist in dark, hot, and pressurized rock cracks, then estimates about where life is possible must also expand.

This is why the question posed by Karen Lloyd, a geomicrobiologist at the University of Southern California, is critical: “If there is a worm down there, what else are we missing?”