Aer Lingus Flight 712
On Aer Lingus Flight 712 in 1968, 61 passengers and crew died while flying from Cork to London. There was never a true cause linked to the accident. Eventually, 14 bodies were discovered, but there were no survivors.
B47 Stratojet
This is a weird one. This involves a plane the US government was carrying nuclear weapons on. In 1956, a B47 Statojet was lost while it was flying over the Mediterranean. Nothing from the flight was ever recovered. No bodies, no crew, no debris, and no missiles. During this flight 2 of a total 11 nukes in the history of the US were lost.