Tickets sold out in 10 minutes for what is quickly becoming Warren County’s most popular attraction.Taping for Season 5 of the Bluegrass Underground TV series on PBS is this week at Cumberland Caverns with hundreds of visitors preparing to converge on Warren County from around the globe.“We sold tickets to people in 37 states and six countries,” said Bluegrass Underground creator Todd Mayo. “More than 99 percent of the people attending these shows are not from Warren County.”Mayo came up with the idea of Bluegrass Underground during a family visit to Cumberland Caverns. It first started as a radio show recorded in the Volcano Room of the cave before turning into an Emmy award-winning TV series that airs in some of the biggest U.S. markets, including New York and Los Angeles.“More and more people are seeing the TV show and putting this on their bucket list,” said Mayo.Chamber of Commerce office manager Norma Sterchi Clark estimates the Chamber gets more phone calls about Bluegrass Underground than any other event.Bluegrass Underground will have more than 20 performances at Cumberland Caverns in 2015, but this is the biggest of them all with 12 bands scheduled to play over three days beginning this Friday.“Even though we’ve done this four times before there’s still an unknown element,” said Mayo.