Charleston has a mystery on its hands.

Theories abound but no one has identified the source of a thunderous explosion beneath Charleston Harbor that sent a 30-foot-wide spray of water 8 feet into the air Tuesday, rocking the City Marina's dock office hundreds of feet away and causing numerous boaters to fear that their vessels had just been rammed.

The rumor mill is churning. Some folks suggest that the blast could have come from an old explosive that sat dormant for years. Others wonder if it was a result of a geological hiccup of sorts. Maybe even a random propane or scuba tank.

But a day later, authorities still don't know what happened.

"We have no idea what it is," Coast Guard Petty Officer 1st Class Chris McDonald said.

Officials from numerous federal, state and local agencies responded to the harbor shortly after the call came in about 2 p.m.

Officers used sonar and sent divers 31 feet down to the harbor floor in search of clues but found nothing to indicate any past or future dangers, according to Charles Francis, Charleston police public information officer.

Robert Freeman, managing partner of the City Marina Co., has heard all of the rumors. He suggests that the culprit was something a bit more ordinary.

Freeman said divers recovered pieces of rusted metal from the harbor, leading him to believe that an old propane or scuba tank finally rusted through and ruptured under the pressure.

"It could have been a situation where someone dropped a tank overboard," Freeman said. "That's the most plausible thing I've heard so far."

Francis confirmed that rusty pieces of metal were found in the water, but said investigators haven't determined what the metal is from.

Freeman said the spot where the marina is now served as a naval base in the 1920s and 1930s. When the city took over the property a couple of decades later, it came with a mine storage facility, leading some to wonder if one of those mines or another type of explosive made its way to the bottom of the harbor.

Others have wondered about some type of earthquake- related event, but there was no seismic activity in the area Tuesday, according to the U.S. Geological Survey.

South Carolina Electric &Gas confirmed for police that none of its equipment was responsible for the incident.

A Charleston police report released Wednesday sheds some light on what it was like to be among the multimillion- dollar yachts docked at the marina when the explosion happened.

A marina employee told police he was in the dock office about 500 to 700 feet away when a loud boom shook the office.

"It felt like a boat or something had run into the dock," Freeman said Wednesday. "There was a lot of percussion."

Crew members aboard the 156-foot yacht Themis described to police an 8-foot "bubble" they estimated to be about 30 feet in diameter, located just about 25 feet from the vessel.

Witnesses said mud from the harbor floor came up from the bottom.

Benjamin Whitney, captain of the Themis, owned by Lowcountry litigator Ron Motley, told police he thought something had struck the vessel. He and the crew checked the ship for damage but found nothing. They also noticed that none of its emergency alarms activated.

Authorities set up a "safety zone" surrounding the area while they searched for clues, but reopened the area after divers found nothing presenting a danger.

Francis said the incident still is under investigation.