Once he arrived at the Dead Sea, Costigan, having run out of his supplies of fresh water, resorted to drinking the water of the sea, leading to further dehydration and fever. Before his illness incapacitated him, he spent several days sailing back and forth about the sea, taking depth soundings. Eventually, too ill to continue, he was taken by his mate to a local monastery and sent from there by donkey to Jerusalem for care. He died along the way and was buried on Mount Zion.