Towards the end of the French ancien regime, Jean François de Galaup, comte de Lapérouse, was tasked with circumnavigating the globe in an attempt to both duplicate the feat of Captain Cook, and add to the monarchy's understanding of the world. His two ships -- the Astrolabe and Boussole -- never returned to France, though logs from the first half of the expedition made their way back to France through their Russian interpreter, who disembarked in Russia.
Later expeditions concluded that Laperouse's fleet came to grief off Vanikoro, an island in French Polynesia. My question is: how do we know this, and how certain are we of it?
[Edit to add an interesting side-fact: a young officer named Napoleon Bonaparte applied to join the doomed expedition. He was turned down.]
La Perouse was last seen when he and his expedition left Botany Bay (Australia), after a six week stay, on 10 March 1788. Relics of his expedition were later found on Vanikoro by the British Captain Peter Dillon. On hearing of this while visiting Tasmania, the great French explorer Dumont d'Urville diverted his expedition to the New Hebrides where he confirmed it by collecting additional artefacts and returning them to France.
TLDR: We know because confirmed wreckage from the expedition was found there by two different ships but we don't have details of exactly what happened.