SILVERDALE, Wash. (AP) — The U.S. Navy says the mine found floating in Puget Sound Tuesday was an inert mine used during a 2005 exercise at its Naval Undersea Warfare Command in Keyport.

The Navy said Thursday that the exercise was a chance for academics to demonstrate how unmanned underwater vehicles detect objects and avoid obstacles.

The object was reported Tuesday floating in waters between Brownsville and Bainbridge Island. Coast Guard and Navy bomb technicians detonated later that night.

The Navy says inert training mines were placed in areas near Brownsville, Keyport and Bainbridge Island during the 2005 exercise. But not all of them floated to the surface so some were not recovered.

Officials say they will survey those areas to try to recover others.