The Martian Canals

The Martian canals were a network of gullies and ravines that 19th century scientist mistakenly believed to exist on the red planet. The canals were first “discovered” in 1877 by Italian astronomer Giovanni Schiaparelli. After other stargazers corroborated his claim, the canals became something of a phenomenon. Scientists drew detailed maps tracing their paths, and soon wild speculation began on their possible origins and use. Perhaps the most absurd theory came from Percival Lowell, a mathematician and astronomer who jumped to the bizarre conclusion that the canals were a sophisticated irrigation system developed by an unknown intelligent species. Lowell’s hypothesis was widely discredited by other scientists, but it was also popularly accepted, and the idea managed to survive in some circles well into the 20th century.
How it was Proven Wrong:
Quite unspectacularly, the Martian canals were only proven to be a myth with the advent of greater telescopes and imaging technology. It turned out that what looked like canals was in fact an optical illusion caused by streaks of dust blown across the Martian surface by heavy winds. Several scientists had proposed a similar theory in the early 1900s, but it was only proven correct in the 1960s when the first unmanned spacecraft made flybys over Mars and took pictures of its surface.