Antarctica. Picture desolate plains of ice, lifeless, bitten by a fierce, bitter wind.
Although ninety-eight percent of Antarctica is covered by thick ice, there are bare patches where the rocky surface shows through. These so-called "dry valleys" are some of the most inhospitable places on Earth. Air temperature is well below freezing, and it's dry as a desert because arid, hurricane-force winds suck the snow and other moisture away.
What does survive?
If you were a microorganism in one of Antarctica's dry valleys, where would you hide to escape the biting, arid wind?
For a cryptoendolith, the answer is inside a rock. These Antarctic cryptoendoliths are a type of lichen whose very name translates as "hiding inside rocks." Even in this cold, dry land, these lichen need a certain amount of warmth and moisture to survive. How does living inside a rock help them keep warm and wet?
How do they stay warm and wet?
These lichen live a fraction of an inch inside pieces of porous sandstone. They prefer north-facing stones, which absorb sunlight and heat up during the day.
Even if the air stays bitterly cold, these sunbaked rocks can warm enough to melt whatever tiny patches of snow have escaped the thirsty wind. The water is quickly absorbed by the porous rock, and it can stay inside for weeks-freezing and re-melting as the rock warms and chills.
Of course the lichen freeze and thaw too, as much as several times an hour during a particularly rough day. But they get just enough heat, and just enough moisture, to stay alive. Which is what survival is all about anyway.