How far below Earth’s surface can bacteria survive?

 “Look deep into nature, and then you will understand everything better.”
~~Albert Einstein

So far, we know bacteria can survive 2.8 kilometers (1.7 miles) below the surface. In 2006 researchers discovered several types of subterranean bacteria surviving in ground water deep below a gold mine in South Africa. This water had not been in contact with Earth’s surface for 3 - 25 million years. Most life on Earth gets energy directly or indirectly from the sun. The absolute isolation of this water means the bacteria in it had to get energy from a different source. The most abundant type of bacteria found in the water, called Desulforudis audaxviator, relies on a unusual energy source – radiation from rocks.

Radioactive minerals in its environment break down water and release highly energetic hydrogen gas. The bacteria are able to collect energy when the hydrogen reacts with sulfate. Other bacteria found in the sample use the use the chemical waste from the Desulforudis audaxviator as food.

Its seems possible that Desulforudis audaxviator could have survived no matter what had happened on Earth’s surface during those millions of years. If that’s the case, then this bodes well for the possibility of life below the surface of Mars or perhaps below the icy surface of Jupiter’s moon Europa.

Where did Desulforudis audaxviator get its great name?
In part from the from Jules Verne's Journey to the Center of the Earth. In the story a secret inscription in Latin reads, “Descende, audax viator, et terrestre centrum attinges.” In English it reads, “Descend, bold traveler, and you will attain the center of the earth.”