Among the improvements recommended by a 1930s board headed by Rear Admiral Arthur Japy Hepburn in the run-up to World War II was a breakwater upon Luminao Reef to the west of Cabras Island. Construction began in August 1941. Limestone blocks quarried at Cabras Island were skidded along the reef to an improvised derrick. By the time of the Japanese invasion in December 1941, one mile of breakwater, 36 feet wide five-feet above sea level had been constructed. The Glass Breakwater, named after the captain who captured Guam in 1898, was extended on Calalan Bank to a width of . The northern arm of Apra Harbor, from the main island through Cabras to the tip of the breakwater at Spanish Rocks measured .