An extraordinarily loud boom shook parts of South Carolina on Thursday, with witnesses across the state reporting a noise like a deafening thunderclap or a huge explosion. But what caused it? While residents and internet sleuths quickly came up with theories, experts just as quickly dismissed them, leaving a mystery that lingered a day later. About 5:30 p.m. Thursday, the boom was heard throughout the central part of South Carolina, including the capital city, Columbia, and an area extending at least 40 or 50 miles east. The U.S. Geologic Survey said it was centered just about 3 1/2 miles northeast of St. Andrews, in the Columbia metro area. There were no reports of damage or serious injury, but it left the region puzzled. Hundreds of residents described hearing the sudden, jarring noise, and a rumble that felt like an earthquake.…