An unknown US Nuclear Submarine arrived today, and tied up at Shearwater.
Nuclear submarines are restricted to Shearwater, and Diesel's will come right into the Dockyard. Radio traffic referred to the boat as "US Naval Unit" typically us warships will be referred to by their hull number - "US Warship 52". She is likely an Improved Los Angeles Class Submarine based on the sail shape, and lack of diving planes on the sail, but this is unconfirmed.
She arrived in, and stayed close to the fog, so here is an enhanced photo:
And the original: