Crew members aboard some of the luxurious yachts at the Charleston City Marina on Tuesday afternoon reported a large underwater explosion that rose some 6 feet above the waterline.
Hours later, officials still didn't have a clue as to the cause.
Authorities from multiple agencies quickly responded to the marina about 2 p.m. after at least one boat crew member reported a plume of water and mud that was about 30 feet wide.
Boaters were kept out of the area as Charleston County sheriff's deputies and Charleston police officers slowly cruised along looking at sonar readings of the harbor floor.
Authorities said they don't know what caused the explosion and weren't about to speculate. By 6:30 p.m., all the divers were out of the water and the Coast Guard had reopened the area to boat traffic.
Jeff Cox, a crew member aboard the 124-foot vessel Endless Summer said he didn't see the explosion but he heard it. He immediately ran on deck and saw the displacement of water and silt about 30 meters from where the boat was docked on the marina's MegaDock.
The investigation is continuing.