Their abduction by extraterrestrials. According to the Hills (pictured), during a drive back home to Portsmouth, N.H., following a vacation in Canada, at around 10:30 p.m., they encountered a strange light in the sky. The couple claimed the light was actually an alien spacecraft, upon which they were forcibly taken. The incident was the first widely publicized account of alien abduction in the U.S.
Click here for a brief but thorough account of the incident at Wikipedia.