What happened
In March 1966 hundreds of sightings were reported over southern Michigan. On March 20, farmer Frank Mannor of Dexter, together with his son and several police officers, watched a luminous object over the marshes, and the next evening, March 21, dozens of female students at Hillsdale College spent several hours watching lights circling over a swamp next to the campus. Astronomer J. Allen Hynek, an adviser to Project Blue Book, was sent to Michigan. At a press conference on March 25 he suggested that some of the lights may have been burning swamp gas. The explanation drew a wave of ridicule in the press, and Michigan congressman Gerald Ford demanded congressional hearings, which led to the creation of the Condon Committee.
Witnesses
Frank Mannor and his family, Washtenaw County police officers, Hillsdale College students and the college dean, and hundreds of residents of the region.
Explanations and hypotheses
Hynek stressed that “swamp gas” was meant to apply only to two specific sightings, not to the whole wave. Spontaneously igniting marsh gases are a rare and poorly documented phenomenon, so the explanation did not convince many witnesses. Hynek himself later recalled the episode as a turning point that led him to take UFOs more seriously. Many other sightings from the wave were attributed to stars, planets, and aircraft.
Disputed A likely explanation exists, but some of the facts are still disputed.
More: Wikipedia (en)