Egyptair Flight 990
In 1999, Egyptair Flight 990 from New York to Cairo crashed into the Atlantic Ocean killing all 217 passengers and crewmates on board. No one really knows what happened.
BSAA Avro Lancastrian Star Dust
In August 1947, Star Dust vanished as it flew between Buenos Aires, Argentina, and Santiago, and Chile. There were 11 people missing for over 50 years. Naturally, many theories arose about what happened including conspiracy theories about an alien abduction. The remains of 9 people were finally found at the foot of a glacier in the Andes. No further information is known.