This vision of lands and seas similar to Earth’s spurred speculation about the habitability of Mars. In his book, Proctor wrote: “Processes are at work out yonder in space which appear utterly useless, a real waste of Nature’s energies, unless, like their correlatives on earth, they subserve the wants of organized beings.”

Other Mars geographers, known as areographers, continued in this vein. The maps of French astronomer Camille Flammarion, such as the one (below) from his 1884 book Les Terres du Ciel, resemble Proctor’s, though they’re more detailed. Like Proctor, Flammarion believed the planet could support life. In 1873, he wrote in La Nature: “On earth the smallest drop of water is peopled with myriads of animalcules, and earth and sea are filled with countless species of animals and plants; and it is not easy to conceive how, under similar conditions, another planet should be simply a vast and useless desert.”

All this speculation about life on Mars influenced how subsequent maps of the planet were interpreted. In the late 1870s, Italian astronomer Giovanni Schiaparelli began making maps based on his observations of Mars (below) that looked very similar to Flammarion’s. But by 1891, the presumed waterways separating landmasses on his maps had become much straighter (top of post). People couldn’t resist interpreting the lines as the work of intelligent beings. They just looked too straight to be natural.

Though he didn’t completely rule out the possibility that the lines could be artificial, Schiaparelli was skeptical. But it seems he inadvertently fanned speculation by describing the lines on his maps with the Italian word canali. This was translated into English as “canals,” which are artificial waterways by definition. In Italian, however, the word is also commonly used to mean natural channels, and this appears to be the meaning that Schiaparelli had in mind.

“It is not necessary to suppose them the work of intelligent beings,” he wrote in 1893. “And, notwithstanding the almost geometric appearance of all of their system, we are now inclined to believe them to be produced by the evolution of the planet, just as on the Earth we have the English Channel and the channel of Mozambique.”

The names Schiaparelli gave to locations on Mars largely survive today, but his most prominent legacy is the canali. Their enduring popularity was thanks largely to American astronomer Percival Lowell, who embraced and popularized the idea that the straight lines on Schiaparelli’s maps were water conduits built by Martians. In a lecture to the Boston Scientific Society in 1894, Lowell said, “The most self-evident explanation from the markings themselves is probably the true one; namely, that in them we are looking upon the result of the work of some sort of intelligent beings.” Lowell made a map (below) in 1897 that is filled with dozens of oases connected by more than 200 canals, each of which he named.

Lowell correctly anticipated that his scientific colleagues would be reluctant “to admit the possibility of peers” inhabiting other planets. But the public loved the idea, and his lectures were often packed. Lowell himself was so taken with the theory that he built an entire observatory in Flagstaff, Arizona, and spent years observing Mars and mapping its surface.

Soon, however, criticism of the canal hypothesis mounted as scientists learned more about Mars. There were the prohibitively cold surface temperatures that would rule out running water, and a scientific demonstration proved the straight lines could be merely the result of an optical illusion. But Lowell continued to write popular science books, culminating with Mars as the Abode of Life in 1908. His ideas persisted in the popular imagination for decades and inspired countless works of science fiction, including the 1938 “War of the Worlds” radio broadcast that dramatized an alien invasion and inadvertently fooled some listeners into believing it was real.

The canals hung around on maps as well, including, surprisingly, a map (above) made in 1962 for the U.S. Air Force. The map, which looks a lot like some of Lowell’s and Schiaparelli’s renderings, was made by Earl Slipher, an American astronomer who had joined Lowell Observatory in 1908. The Air Force used Slipher’s map in the planning of the first Mariner flyby missions. When the Mariner 4 spacecraft had the first close encounter with the Red Planet in 1985, any lingering doubts about the canals were finally put to rest.