Bacteria-eating worms dwell in treasure caves deep below the earth's surface, where only single-celled organisms were thought to exist. Although it may sound like the premise of a sci-fi masterpiece, this scenario is the major finding of a recent report in Nature. The report describes the discovery of four species of nematodes, or roundworms, in the depths of South African gold mines, where conditions could charitably be called inhospitable. The worms were recovered from fracture water used to flush mine boreholes at depths of 0.9–3.6 km. In the boreholes where the worms were found, ambient temperatures were 24–48 °C, well above the tolerance range of most terrestrial nematodes. The environments were also hypoxic, with aqueous O2 concentrations ranging from 13 μM to 72 μM.

One of the species recovered by the research team, which was led by Gaetan Borgonie (Ghent University, Belgium) and Tullis Onstott (Princeton University, NJ), has never before been described. Because it was discovered deep underground, the new nematode was named Halicephalobus mephisto, alluding to the Faustian lord of the underworld Mephistopheles. H. mephisto is roughly 0.5 mm long with a ringed body and relatively long tail (Nature 474, 79–82; 2011). The known species Plectus aquatilis was recovered from a different mine, along with a monhysterid nematode. DNA of a fourth nematode, also a monhysterid species, was recovered from a third location.

All three living species preferred to feed on indigenous bacteria from the mines rather than Escherichia coli. The native bacteria typically exist in biofilms that require time to become established, suggesting that there is a functioning ecosystem at depth in the mines.

Scientists are now wondering how widespread such communities are and how long they have existed. Borgonie, Onstott and their colleagues hope to continue investigating the complexity of underground ecosystems and to sequence the genomes of the recovered worms, looking for clues as to whether the worms have evolved in the subterranean environments. Onstott spoke with Nature News about the future research. “That could tell us a lot about evolution,” said Onstott. “Is H. mephisto endowed with any special capabilities? Is it more primitive? Has it acquired attributes that imply adaptation and evolution in the subsurface?”

The current results show that deep underground ecosystems are more complex than previously thought. They also suggest the possibility that similarly complex communities could exist in subsurface environments in other locations on earth, or even on other planets in our solar system.