For the first time, scientists have found complex, multicelled creatures living a mile and more below the planet's surface - raising new possibilities about both the spread of life on Earth and potential subsurface life on other planets and moons.
Nicknamed "worms from hell," the nematodes, or roundworms, were found in several gold mines in South Africa, where researchers also have made breakthrough discoveries about deep subterranean single-cell life.
The two lead researchers, Gaetan Borgonie of the University of Ghent in Belgium and Tullis Onstott of Princeton University, said the discovery of creatures so far below ground, with nervous, digestive and reproductive systems, was akin to finding "Moby Dick in Lake Ontario."
"This is telling us something brand new," said Onstott, whose pioneering work in South Africa over the past decade has revolutionized the understanding of microbial life known generally as extremophiles, which live in places long believed to be uninhabitable.
"For a relatively complex creature like a nematode to penetrate that deep is simply remarkable," he said.
An article introducing the subterranean nematodes, one of which formally was named Halicephalobus mephisto after the "Lord of the Underworld," appears in Wednesday's edition of the journal Nature. H. mephisto was found in water flowing from a borehole about one mile below the surface in the Beatrix gold mine.
The research is likely to trigger scientific challenges and cause some controversy because it places far more complex life in an environment where researchers generally have held it should not, or even cannot, exist.
Borgonie led the South African nematode investigation largely without professional support or funds. He contacted his future partner with a cold call, and Onstott - who began his own deep mine work with similarly limited funds and amid similar professional skepticism - was both intrigued and inclined to help a fellow risk-taker.
Borgonie said that while nematodes are known to exist on the deep ocean floor, they generally have not been found more than 10 to 20 feet below the surface of the ground or the ocean bed. However, he saw no reason why they wouldn't be found farther down. The nematodes he ultimately discovered live in extremely hot water coming from boreholes fed by rock fissures and pools.
In addition to uncovering a new realm of biology on Earth, Borgonie and Onstott wrote that the nematode findings also could have important implications for extraterrestrial research, or astrobiology.
Scientists seeking life beyond Earth are intrigued by the possibility that microbes could be living below the surface of Mars, in particular - a planet that is now cold, dry and bombarded by harmful radiation but was once much wetter, warmer and better-protected by an atmosphere.
"What we found shows that harsh conditions do not necessarily exclude complexity," Borgonie said.
He said that if life did originate on Mars and if it had sufficient time to go underground deep enough to survive worsening conditions, "then evolution of Martian life might have continued underground.... Life on Mars could be more complex than we imagined."
Carl Pilcher, director of NASA's Astrobiology Institute in California, said that the nematode discovery illustrates the usefulness of research on Earth for learning about possible extraterrestrial life.
"It is entirely plausible, in fact extremely likely, that subsurface environments like those described in these papers exist on other worlds in this solar system and in other planetary systems," he said of the new work and Onstott's earlier discoveries.
At least one of the bacteria species discovered earlier by Onstott and Lisa Pratt of Indiana University lives entirely disconnected from anything on the Earth's surface or produced by photosynthesis. It uses the radioactive decay of nearby rocks as the energy source to break apart molecules that it then feeds on.
Borgonie speculates that the nematodes, which feed on bacteria, traveled through the cracks and crevices of the rock in search of food. While they were determined to have lived deep underground for 3,000 to 10,000 years, the bacteria discovered by Onstott was found to have lived at its great depth between 3 million and 40 million years. A major difference between the two appears to be that while the nematodes adapted, the bacteria have evolved.
Complete worms, up to one-third of an inch in length, were found in two mines, and DNA of another was found in a third. They were found in water flowing from boreholes in the rock of the mines at depths from two-thirds of a mile to more than two miles. The worms nearer the surface were brought to a lab and survived, while the specimen at the deepest level was a DNA sample from a nematode but otherwise impossible to identify.
Research into the distribution of underground microbes in recent decades has led scientists to conclude that more than half of the biological mass on Earth is below the surface.