For human travelers, the iconic moment of space exploration occurred a half-century ago, when Neil Armstrong planted the first human boot-print on the moon. But if you don't mind using robots as our stand-ins, the greatest era is unfolding right now on Mars, where NASA's Curiosity rover is rolling across the rusty, dusty surface and leaving behind tread marks that spell out the letters "J-P-L" in Morse code. JPL stands for the Jet Propulsion Laboratory, the NASA center that designed and built Curiosity along with three earlier Mars rovers. Collectively, these machines have racked up 46.4 miles of travel, tremendously expanded our understanding of the Martian environment, and energized the search for life in the universe. Everywhere the rovers have gone, they have discovered unexpected complexity.