The HD-9 was the submarine that Matthais Louvier (Rouvier in the European version) died in. In Endless Ocean 2, it is said that its engines fail trapping Matthais inside. He got out and attempted to swim up, but he disappeared. Inside of the sub is a waterproof bag, which holds the third piece to the Dragon Flute and his last letter. The HD-9 is found inside Osiris's Courtyard in the Zahhab Region Depths.