To date, wherever life has been sought on Earth, it has almost always been found—from high in the stratosphere (Imshenetskii et al., 1975, 1978, 1986; Wainwright et al., 2003) to deep in the ocean trenches (Takamia et al., 1997; D'Hondt et al., 2004) and even within the Earth's crust itself (Pedersen, 2000). Microorganisms have also been found in some of the most extreme environments. They have been found to exist in ice, boiling water, acid, salt crystals, toxic waste and even in the water cores of nuclear reactors (Rothschild and Mancinelli, 2001).

Antarctic subglacial lake ecosystems have the potential to be one of the most extreme environments on Earth, with combined stresses of high pressure, low temperature, permanent darkness, low-nutrient availability and oxygen concentrations derived from the ice that provided the original meltwater (Siegert et al., 2003), where the predominant mode of nutrition is likely to be chemoautotrophic. Yet, to date, the identification of significant subglacial bacterial activity in the Arctic, beneath glaciers (Skidmore et al., 2000, 2005) and in subglacial lakes (Gaidos et al., 2004), as well as extensive work on permafrost communities and work in the deep sea, suggests that life can survive and potentially thrive in these types of environment. Microbial life has been shown to function at gigapascal pressures (Sharma et al., 2002) and bacteria recovered from the deep ocean at around 4000 m have been shown to retain both structural integrity and metabolic activity. They have shown activity in the Antarctic at −17 °C (Carpenter et al., 2000) and to exist in the pore spaces between ice crystals (Thomas and Dieckmann, 2002).

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