The Great Sound was also the landing area used by Royal Air Force and airline flying boats operating from Darrell's Island, and Fleet Air Arm seaplanes operating from the RNAS Boaz Island. As part of the Destroyers-for-bases deal between the US and the UK which granted 99-year leases to the US on parts of the island, the US Navy began construction of a third flying boat air station, combined with naval docklands, called Naval Operating Base Bermuda,[8][9] on the Great Sound, prior to the US entry into the war. During the course of the war, when Allied trans-Atlantic shipping was organised into convoys, Bermuda became a forming up point for convoys (coded BHX, these joined at sea with HX-coded convoys from Halifax to complete the passage to Europe), with large numbers of vessels anchoring on the Northern Lagoon and the Great Sound.