On 23 June 1985 , the Boeing 747-237B serving Supply the flight Self_motion ( 21473/330 , registration , `` Emperor Kanishka '' ) disintegrated Destroying in midair en route from Montreal to London , at an altitude of 31,000 feet ( 9,400 m ) over the Atlantic Ocean , as a result of the explosion Catastrophe from a bomb Terrorism planted Placing by Canadian Sikh terrorists Terrorism .