The maximum ocean depth so far reported is about 11 000 m, and is located in the Mariana Trench in the Western Pacific Ocean. The hybrid unmanned underwater vehicle, Haidou, is developed to perform scientific survey at the deepest parts of the Earth oceans. For vehicles working at the full-ocean depth, acoustic positioning is the most effective and popular method. The 11 000 m class acoustic positioning system is relatively massive and complex, and it requires specialized research vessels equipped with compatible acoustic instruments. As a compact testbed platform, it is impractical for Haidou to carry an LBL/USBL beacon with its large volume and weight. During the descent to about 11 000 m, horizontal drift could not be eliminated because of the hydrodynamics and uncertain ocean currents in the sea trials. The maximum depth recorded by Haidou is 10 905 m, and determining the precise location of the deepest point is challenging. With the bathymetric map produced by a multibeam sonar, the terrain contour matching (TERCOM) method is adopted for terrain matching localization. TERCOM is stable in providing an accurate position because of its insensitivity to the initial position errors. The final matching results show the best estimate of location in the reference terrain map.