A Korean War-era bomb that rested in shallow Gulf waters just off the coast of the Middle Keys for years until recreational divers discovered it last month was blown up by a Navy ordnance disposal unit on Wednesday.

The explosion was small, sending a plume of water about 25 to 30 feet in the air.

''We are now relatively certain it was a training bomb,'' Navy Lt. John Stewart said. ``But we had to assume the worst case, that it was a live bomb with the potential of 400 pounds of explosives.''

To protect people from flying debris, the area within 6,000 feet of the bomb was cleared. That included closing U.S. 1 on Duck Key for about 30 minutes.

''We thought the bomb was inert, but it was covered in sea growth so we couldn't be certain,'' Stewart said. ``And even with practice bombs, they have small charges in them to shoot a plume of smoke in the air to let the pilot see where he hit the target.''

It was an M117-series bomb, an unguided, general-purpose, 750-pounder that also was used in Vietnam and the Gulf War and is still part of the military's inventory.

Stewart said it was not surprising to find such a weapon in Keys waters.

''There used to be all kinds of bomb ranges down here in the '60s and '70s,'' Stewart said. ``Small planes practiced dropping bombs all the time.''

Divers found the bomb in Channel Key, about 12 miles north of Marathon and about 5,000 feet from U.S. 1.

Before 10 pounds of explosives were set off for the main detonation, Stewart said, a small block of plastic explosives was used to scare away wildlife and fish.

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