Query: What did the US begin building in 1941?
Context: 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.