Yup, seems to be a MKB training mine that was left unswept after an excercise 13 years ago that caused the hubub around Seattle this week.
From U.S. Navy Region Northwest:
The Navy conducted additional analysis on Tuesday’s incident involving an unknown mine in the Puget Sound and subsequent detonation at 8 p.m.
It was determined the mine was from an exercise Naval Undersea Warfare Command, Keyport conducted in 2005. This exercise was an opportunity for academia to demonstrate various Unmanned Underwater Vehicles and their capability to detect underwater objects and avoid submerged obstacles.
During this exercise, inert training mines were placed in areas between Brownsville, Keyport, and Bainbridge Island. Only a small number of the training mines were positively buoyant. Not all training mines were recovered.
It has been confirmed the device destroyed Tuesday was a positively buoyant, inert training mine used during the 2005 exercise.
In order to avoid similar incidents in the future, the Navy will survey the exercise areas and recover any remaining positively buoyant mines.
By request of the State of Washington in the interest of public safety, Navy Explosive Ordnance personnel safely disposed of the device that appeared to be a dated military mine in waters between Keyport and Bainbridge Island, Washington.
The device was detonated at 8:04 p.m. (Tuesday).
The detonation did not create a secondary explosion which indicated the device was inert.
The Navy thanks the following partner agencies for their support in the response: The U.S. Coast Guard, the Suquamish Tribe, State of Washington, Kitsap County, and the National Oceanographic and Atmospheric Administration.