Sandeep Ravindran in Smithsonian:
Ancient bacteria from nearly two miles below Earth's surface: that's what first drew Tullis Onstott to begin his search for life in the most unlikely of places. The geomicrobiologist had just attended a 1992 U.S. Department of Energy meeting about rocks estimated to be more than 200 million years old—older than most dinosaurs. These prehistoric rocks had been unearthed from a gas exploration well, and they turned out to be teeming with bacteria. “That was pretty amazing to me,” says Princeton University's Onstott. “The idea that these bacteria had been living in these Triassic rocks since they were deposited at a time prior to the age of the dinosaurs, that idea caught my fancy,” he says. These rocks were among the first substantial evidence that life existed miles underground, and they jumpstarted researchers’ efforts to study life in the so-called deep subsurface. Over the past 20 years, Onstott and others have found that there is a greater variety of life in a lot more inhospitable places than anyone had imagined.
Deep life has been found all over the world and under a variety of conditions—in oil fields and gold mines, beneath ice sheets in Greenland and Antarctica and in sediments and rocks below the ocean floor. These places can be extremely hostile environments, with pressures 10 to 100 times that at the surface. Temperatures can range from near freezing to more than 140 degrees Fahrenheit. A mile or more below the surface there's no sunlight and very little oxygen. In these austere environments, creatures have to scratch out a living on whatever energy they can muster from their surroundings. This means that the pace of life down there can sometimes be incredibly slow. These microbes can be a thousand- or million-fold less abundant than their brethren above ground. And some may have been around for hundreds, thousands or even millions of years—real microscopic Methuselahs.