Bomb technicians from the Navy detonated a "dated military mine" on Tuesday that was found floating for hours in Washington's Puget Sound between Brownsville Marina and Bainbridge Island.
The Navy said in a statement the object was reported about 2 p.m., and an inspection showed the mine had decades of marine growth on it. Officials said they detonated the mine at 8:04 p.m.
"The detonation did not create a secondary explosion, which indicated the device was inert," the Navy said in a statement. "Its origin remains undetermined. The Navy will continue to investigate."
Video from KIRO-TV shows the moment the bomb was detonated.
The discovery of the device prompted the Coast Guard to set up a 1,500-yard safety zone before the Navy inspected the device and towed it out to a safe area for detonation. The Coast Guard said the ordnance was originally located by a Washington State Department of Natural Resources crew.
Authorities also asked residents along the waterfront to stay inside and away from beaches as a precaution.
"It's just an unusual situation, not many times you see a World War II mine floating around Brownsville Bay," one resident told Q13 News as he checked out the situation.
Brownsville is located a few miles south of Naval Base Kitsap — Keyport's torpedo testing range.
The Associated Press contributed to this report.