Hundreds of feet long, the submarine bobbed near Fort Sumter surrounded by a slew of Coast Guard boats.

The Coast Guard had established a 500-meter security zone around the craft Monday in Charleston Harbor, which had no identifying markings. Three tugboats surrounded the sub.

The Coast Guard had nothing to say publicly about the submarine, and referred questions to Joint Base Charleston. Air Force Lt. Susan Carlson said the sub is the USS Daniel Webster. It was being towed to Virginia for a year of routine dry-dock maintenance, Carlson said.

The Daniel Webster is one of the two decommissioned subs moored at the Charleston Naval Weapons Station that are used to train sailors in the operation of nuclear reactors and nuclear propulsion.

The ballistic missile sub was launched in 1963 and decommissioned in 1990. The USS Sam Rayburn is also used as a training ship for the Navy nuclear sub program in Goose Creek.

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