Answer: Baltic Sea
Notes: Sweden's recent NATO accession, following Finland's entry, completes the alliance's presence around the Baltic Sea, except for Russia. This move, dubbed the 'NATO lake,' aims to bolster regional security. Despite this, Russia, stationed in the heavily-armed Kaliningrad exclave, poses a continued threat, both above and below the water surface. The strategic significance of the Baltic Sea underscores the importance of Sweden's membership in countering potential threats in the region.
