Those who think there could be life elsewhere in the solar system talk of the possibility of bacteria buried in the Martian icecaps, perhaps, or under the icy crusts of Jupiter's moons Europa or Callisto. One problem with such theories is that these are extremely hostile environments with pressures much higher than normal. Similar high-pressure zones on Earth — under the Antarctic ice, for instance — are suitable only for specially adapted organisms known as extremophiles.

The most extreme conditions tolerated by extremophiles are found one mile down, on the ocean floor, at cracks in the Earth's crust called hydrothermal vents. … Organisms there must live without any light, at pressures 600 times greater than atmospheric pressure and tolerate temperatures up to 110 degrees Celsius.