Two or three Japanese submarines were in the area. After establishing the convoy's base course, one released a dummy naval mine in the path of the convoy. When it was sighted by "Underhill" lookouts, the ship's commander ordered a general course change to port. When the last ship had cleared, "Underhill" stood in to sink the mine. After repeated direct hits by the 20-millimeter guns and 30-calibre rifle fire, the convoy realized the mine was a diversionary tactic by the Japanese submarines.