Mars has oceans
Less fanciful than the idea of canals on Mars, but still ultimately proven untrue, was the idea that Mars boasts oceans. In 1784, astronomer Sir William Herschel published a paper on his telescope observations of Mars. He got some things right, or at least close, including an axial tilt of 30 degrees (the actual number is 25.19).
But Herschel did make the mistake of assuming that the dark areas he saw on Mars were oceans, broken up by lighter regions representing land. The idea of Martian oceans would persist throughout the 1800s. Close-up looks at the planet would prove it to be arid, though some scientists think liquid water once flowed on the planet billions of years ago. As of today, the water found on Mars has all been locked up as ice in the soil, with evidence of liquid water at the surface still uncertain.