Photos of an enormous crater on Mars indicate possible underground water—water that could have supported life, and could still be doing so. Images taken by NASA's Mars Reconnaissance Orbiter show the 57-mile-wide McLaughlin Crater, which is now dry. It's one of the planet's deepest spots at 1.3 miles, and minerals there indicate upwelled groundwater may have formed a lake on the spot some 4 billion years ago. One other sign of a former lake, per Space.com: channels that climb 1,650 feet up the walls of the crater's eastern side.
Because the surface of the planet is inhospitably cold, scientists have focused on lower layers in a search for possible life. Earth's underground is home to almost half its living matter, in the form of microbes, and in like fashion, "the deep crust has always been the most habitable place on Mars," says the study's head author. His work began as an attempt to disprove that water had flowed to Mars' surface. But "lo and behold, there was strong evidence for that process in this crater," he notes. "Science is special because we are allowed to change our minds." (Read more space stories.)