In 1967, the largest cave search in US history unfolded in historic Hannibal, Missouri, the boyhood home of author Mark Twain.

Three modern day Tom Sawyers, with no caving expertise but an abundance of bravado, made Hannibal ground zero for a terrifying calamity that would leave its traumatic mark for half a century. Joel Hoag, his brother Billy, and their friend Craig Dowell vanished after exploring a vast and complex maze cave system that had been exposed by highway construction. Fifty years later, their fate remains the ultimate unsolved mystery.