These two sites—a frozen Arctic lake and a toxic tropical cave—could provide clues to one of the oldest, most compelling mysteries on Earth: Is there life beyond our planet? Life on other worlds, whether in our own solar system or orbiting distant stars, might well have to survive in ice-covered oceans, like those on Jupiter's moon Europa, or in sealed, gas-filled caves, which could be plentiful on Mars. If you can figure out how to isolate and identify life-forms that thrive in similarly extreme surroundings on Earth, you're a step ahead in searching for life elsewhere.