S1, E2: It's home to the largest volcano in the Solar System...and a colossal, boulder-strewn valley that would make the Grand Canyon look like a streambed. But despite such otherworldly features, it's the planet most like Earth, and it's the most likely candidate for eventual human habitation. In the past, its mysterious movement in the sky and rumors of a network of canals on its surface made it a source of wild speculation. Did alien life flourish there? Perhaps an advanced civilization that might one day attempt to conquer Earth? Today, NASA orbiters and rovers mapping the planet's surface have put these notions to rest. While they've photographed land features that look chillingly like an enormous human face, so far, they have not discovered intelligent life. Still, the search for biology continues. And with new evidence of vast amounts of frozen water lurking just a few feet below the rusty soil, the possibility of finding at least primitive life on the fourth rock from the Sun seems exceedingly probable.