The planet Mars.
Giovanni Schiaparelli (1877) observed "canali" on Mars. Percival Lowell, a wealthy amateur astronomer, misunderstood "canali" to mean "canal," and thus assumed the canals must be man-(or Martian-)made. Lowell established the Lowell Observatory at Flagstaff, Arizona to study Mars and spent a good deal of his life "mapping" the Martian canal structure.
Of course, the "canals" turned out to be optical illusions, figments of Lowell's imagination triggered by the difficulties of observing Mars from Earth and from Lowell's overwhelming desire to want to find canals and thus life on Mars.
Have you ever wanted something so badly that you actually imagined it to be?
To learn more about Lowell and Mars: