Canadian Pacific Air Lines Flight 3505 was a scheduled international passenger flight taking off from Vancouver, British Columbia, bound for Tokyo, Japan. On July 21, 1951, the Douglas DC-4 aircraft was carrying 37 passengers and crew members when it disappeared over the Pacific Ocean. The last known contact was made shortly after its departure, and the plane's vanishing initiated one of the largest search operations in aviation history.

The search encompassed a vast expanse of ocean, but despite extensive efforts involving aircraft, ships, and trained personnel, no trace of the airplane was ever located. Search operations continued for several weeks, but as time passed without any sign of the aircraft, the mission was eventually scaled back. The incident left families of the missing occupants in prolonged uncertainty, and the mystery surrounding the flight's disappearance generated significant media coverage and public intrigue, questioning aviation safety protocols.

Investigators examined potential causes for the disappearance, ranging from technical failures to navigational errors and environmental factors. However, the lack of recovered wreckage hindered definitive conclusions, and the incident remains one of the unsolved mysteries of aviation's early years, eventually becoming part of the lore surrounding missing aircraft in the Pacific.