UA811
From the NTSB report: "The undersea search operation was begun on July 22, 1990, using the Orion, a state-of-the-art Navy side-scanning sonar “fish.” Searching in the area selected by analysis of radar data and undersea currents, the Orion located a debris field on its first pass over the 14,200-foot-deep ocean floor. The second pass located a significant sonar target, which later analysis indicated was probably the cargo door. Since the Orion is only capable of searching, the debris field was marked with transponders for use during the subsequent recovery phase."
Advantage was that the falling debris had been seen on radar...