Construction began in 1941 of two airbases consisting of 5.8 km2 (2.2 sq mi) of land, largely reclaimed from the sea. For many years, Bermuda's bases were used by US Air Force transport and refuelling aircraft and by US Navy aircraft patrolling the Atlantic for enemy submarines, first German and, later, Soviet. The principal installation, Kindley Air Force Base on the eastern coast, was transferred to the US Navy in 1970 and redesignated Naval Air Station Bermuda. As a naval air station, the base continued to host both transient and deployed USN and USAF aircraft, as well as transitioning or deployed Royal Air Force and Canadian Forces aircraft.

What did the US begin building in 1941?