At the start of the twentieth century, a Mars “craze” struck Earth. Humans went wild with speculation that the red planet might harbor an advanced civilization. Historian of astronomy William Sheehan explains the science that spawned this imaginative idea. It all began with an optical illusion—a network of lines on Mars that some astronomers believed were irrigation canals. Sheehan is the author, with Jim Bell, of “Discovering Mars: A History of Observation and Exploration of the Red Planet.”