As recently as a dozen years ago, writes Richard Monastersky in the March 29, 1997, issue of Science News, life was considered to inhabit only the Earth’s atmosphere and waters along with a thin veneer of its crust. The Russian biogeologist Vladimir Vernadsky used the term “biosphere” in 1926 to represent this limited area of the planet to which life was confined.

Some of the living conditions in the biosphere can be pretty horrific, according to Michael Madigan and Barry Marrs in the April 1997 issue of Scientific American, and are inhabited only by different species of bacteria given the blanket name “extremophiles.” Two types the authors point to are the heat-lovers, or thermophiles, who thrive in boiling water, and their opposite numbers known as psychrophiles, or cold-lovers, found permanently locked in glacier ice.

Elizabeth Pennisi tells of one resilient bacterium in the March 10, 2000, issue of Science that survives in a mix of toxic metals and sulfuric acid at a pH of 0 at the bottom of an abandoned copper mine. But no one who studies extremophiles ever thought they might be found embedded in solid rock far below the planet’s surface. The combination of high temperatures and crushing pressure, along with the lack of nutrients, ruled out any possibility of even the hardiest microbe’s survival. They were wrong.

The discovery of microbes in deep rock came about as a result of test drills sponsored by the Department of Energy in a study on the storage of nuclear materials. Ricardo Guerrero and Lynn Margulis writes in the July-August 1998 issue of The Sciences that deep-dwelling bacteria have been found in rock at a depth of 13/4miles and at a temperature of 167 degrees F.

Monastersky says that bacteria might be found at depths of 21/2miles below the continental crust and 41/2miles below the ocean floor. He quotes Thomas Gold of Cornell University as saying that the mass of all deep-dwelling microbes could equal or exceed that of all organisms above the surface. This extraordinary prediction is borne out to some extent by Robert Cooke’s report in the April 14, 1997, issue of Technology Review that every gram of sediment from a 1,500-foot-deep seafloor coring off the coast of Japan contained countless millions of microorganisms. Cooke says the huge numbers of subterranean bacteria should not come as a great surprise because they were the Earth’s only inhabitants for 3 billion years and had plenty of time to evolve into diverse forms.

Monastersky says that the microbes lead a spartan existence but have learned to survive in either sedimentary or igneous rock. In the former, they scavenge trace amounts of organic materials gluing the rock particles together, while in the latter, they are thought to make organics directly from trapped carbon dioxide, hydrogen and water. In the May 11 issue of Science, Steven Petsch of Woods Hole Oceanographic Institute in Massachusetts reports that the weathering of shale, a type of sedimentary rock, is accelerated by bacteria using an organic binder called kerogen as a carbon source.

Existence of the rock-dwelling bacteria has revived hopes that similar life may be found on Mars. In 1996, NASA officials sent shock waves through the space community when they announced that a meteorite of Martian origin appeared to contain traces of fossil microorganisms. Since then, doubt has been cast on the evidence, but recent indications that Mars once had running water that may still be present in the subsoil, and the fact bacteria exist that could dwell in the harsh environment of Mars, has again raised interest in the possibility.

The rock-dwelling bacteria also lent some support to a controversial theory put forth by British astronomers Fred Hoyle and Chandra Wickramasinghe in their book “Lifecloud” in 1978. In it, the authors claim that life could have been seeded on a primitive Earth by bacteria-laden meteorites or comets. Critics derided them saying that no living creature could have survived the cold of space or fiery entrance into the Earth’s atmosphere.

Whether they could have or not is a moot point, according to physicist Jay Melosh. At a meeting reported on by Richard Kerr in the April 6 issue of Science, Melosh presented a statistical study on the probability that life on Earth could have an extraterrestrial origin. He found that it would take more than a trillion years for such an event to happen, ruling out the possibility that life came from space early in the formation of the solar system.

Clair Wood caught chemistry and physics for more than 10 years at Eastern Maine Technical College.