A bundle of letters written aboard one of history's most famous lost voyages has come to light in Paris, more than two centuries after its author disappeared at sea. The correspondence was written by Jean-André Mongez, a scientist travelling with the French expedition led by Jean-François de Galaup, count of La Pérouse, which sailed from Brest in August 1785 with two ships and 225 people aboard.

The expedition was meant to rival the achievements of Captain James Cook, charting the Pacific and opening new trade routes for France. Instead it ended in catastrophe. After leaving Botany Bay in Australia in March 1788, both vessels, La Boussole and L'Astrolabe, vanished without trace. Wreckage was found in the following decades, but the ships were only formally identified in 2005, leaving the fate of the crew one of maritime history's great mysteries.

Mongez himself died in 1788, before the disappearance. His letters, addressed to Louise Victoire Le Cloustier, the married daughter of a Rouen aristocratic family, were discovered tucked inside a book of engravings by auctioneer Benoît Derouineau. Manuscript expert Ludovic Miran described the find as a miracle, noting how rarely personal documents survive from an expedition remembered chiefly for its scientific aims.

The letters strike a strikingly private note. Mongez writes of his unhappiness at Le Cloustier's marriage and promises to return a better man, while also recording the voyage's violence and grief, including the killing of fellow crew members in Samoa. He questions whether the scientific rewards justified the human cost. Offered by the Daguerre auction house, the letters will be sold as one lot at the Hôtel Drouot on 17 November, carrying an estimate of €30,000 to €50,000.

The sale closes a circle that began with the French king himself. According to tradition, Louis XVI asked after La Pérouse on the morning of his execution in January 1793, still hoping for news of the ships that had never come home.