Radiation typically does not deter microorganisms, either. So long as they are not in the direct pathway of an atomic blast – which would likely burn them up – they can thrive in containers of radioactive waste or near the epicentre of the Chernobyl disaster, for example. Deinococcus radiodurans, one of the hardiest of the radio-resistant microorganisms, has survived trips into space and can endure radiation doses of up to 15,000 gray (the standard measure of the absorbed dose of radiation). For humans, just 5 gray causes death.

Likewise, what we register as deadly chemical environments, some extremophiles call home sweet home. Various organisms depend on arsenic, mercury or other heavy metals for their growth and survival, while others prefer cyanide. In the hot springs of Kamchatka in Russia, menageries of microorganisms metabolise using sulphur or carbon monoxide. “It’s hard to find a chemical that can kill all life,” says Frank Robb, a microbiologist at the University of Maryland.

There might be isolated exceptions, however. Don Juan Pond in Antarctica is the saltiest known body of water on the planet, with salinity levels topping 40%. (The Dead Sea is around 33%). Researchers have recovered traces of microbial life from the pond, but they are still trying to determine whether it is actively growing and replicating there, or just blowing in from other locations. Don Juan counts as “an example of a place on the surface of Earth where we might expect life, but cannot verify the presence of active life,” says Corien Bakermans, a microbiologist at Penn State University.

For now, extreme heat and some synthetic laboratory environments might be the only sterile conditions on the planet’s surface that it’s possible to find zero traces of life. New organisms are regularly discovered that push the boundaries of life as we understand it, although where that line will ultimately be drawn remains unknown. As Santos says, “What does not exist is more difficult to prove than what does exist.”

Even if there are some lone holdouts of sterility in the natural world, however, the environment exists in a constant state of flux – and if extremophiles teach us anything, it is that organisms are always capable of adapting. “Give them enough time and they will find a way,” says DiRuggiero.

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