ABritish possession since the Treaty of Utrecht in 1713, the Rock of Gibraltar occupies an important strategic position at the entrance to the Mediterranean and has a large dockyard and, since 1941, an airfield. During World War II Gibraltar was a vital link on the sea route to Malta and Alexandria and accommodated both an MI5 defense security officer and a Secret Intelligence Service station. It acted as a base for Special Operations Executive activities overland into southern Spain or by caique to southern France, and MI9 received evaders at the frontier for transfer to England.
After World War II the interior of the Rock, extensively tunneled over many decades, provided secure accommodation for a SOSUS terminal that monitored Eastern Bloc submarine transits and surface movements through the natural choke point.