It has been a long time since a US Navy submarine has visited Halifax. Once common during the Cold War, the number of visits dropped off dramatically after the invasion of Iraq following 9/11. There has been enhanced navy activity in Halifax in the last few days and weeks, with several ships exercising and coming and going, so some sort of exercise may be going on.
What I believe to be a Virginia class SSN (attack submarine) arrived this afternoon and was met by a small flotilla of harbour craft, including several Dockyard tugs.
All US subs are now nuclear powered, and thus must tie up at Shearwater (Navy Jetty Alpha) in Eastern Passage.