Percival Lowell wasn’t the first to think he’d discovered life on Mars, but he was among the last. In the late 19th and early 20th centuries, the American astronomer published a series of books promoting his theory that observable features on the surface of the Red Planet were the handiwork of an intelligent species on the verge of extinction. The objects of Lowell’s fascination—and the wider astronomy community’s scorn—were the so-called “Martian canals,” which he believed were used to route water from the planet’s ice caps.
NASA has been robotically exploring Mars since the mid-’60s, and because of these missions we’re now fairly certain that the planet isn’t home to any extraterrestrial engineers. (Sorry, Percy.) But these spacecraft did find an abundance of geological evidence that Mars may once have had