The airplane exploded and crashed into the sea, and everyone on board died.
Literal
Airplane [subject-が] exploded [then-て] sea [into-に] crashed, were-riding all people [subject-が] died.
A grave, news-style sentence chaining three events. 墜落し uses the conjunctive masu-stem (連用中止法) to link clauses in a formal/written register. 乗っていたすべての人 is a relative clause: 'all the people who were riding.' 亡くなる is the standard polite/indirect way to say 'to die.'