The thought of Widespread Panic performing in a Warren County cave would have seemed like a fairy tale just a few years ago.But the “Bluegrass Underground” TV series has transformed Cumberland Caverns into a high-profile concert venue capable of attracting big names.Vince Gill and Old Crow Medicine Show have been two of the biggest acts to perform 333 feet underground in the Volcano Room of Cumberland Caverns. On Monday night, popular jam band Widespread Panic added its name to the list, playing a catchy set to a standing-room-only crowd.“Bluegrass Underground” producer Todd Jarrell said the tickets sold out in 30 seconds with fans from around the country converging on Warren County for the show. One SUV parked outside Cumberland Caverns had a Texas license plate.“Man, what a show,” said Jarrell while getting congratulatory handshakes following a successful production.“Bluegrass Underground” creator Todd Mayo said he hopes Widespread Panic serves as a springboard to get other big-name bands to perform at what he calls one of the most unique concert venues in the world.