(CNN) It's a really unusual day when state authorities have to call the Navy's Explosive Ordnance Disposal team.
But when there's what looks like a mine floating in a bay not far from a marina, that's what you do.
While people on land were told to stay inside, the Coast Guard on Tuesday set up a safety zone and Navy divers went into Port Orchard Bay in Puget Sound in Washington to lasso the big, round, rusted object with metal prongs bobbing in the water.
"Upon initial inspection, the unidentified moored mine was found to have decades of marine growth," the Navy said.
The mine broke loose from its anchor, spokeswoman Sheila Murray said.
The crew towed the mine away for hours while authorities tried to keep residents and news helicopters circling the area away.
They asked residents to take shelter in their homes, keep their pets inside and avoid the beaches.
Just after 8 p.m. local time, the decades-old mine was detonated, sending water rushing up into the air.
It was safer to detonate the mine far from the shore, the Navy said.
The US Coast Guard had established a 1,500-yard safety zone around the unexploded ordnance, Coast Guard spokeswoman Ali Flockerzi said.
The object was discovered by the Washington State Department of Natural Resources.