Answer: Episode title!
In "The Expedition Approximation," Sheldon and Raj venture into the depths of the university basement to determine whether or not they could survive in a salt mine while conducting dark-matter research. Cramped together in close quarters with nothing but their thoughts and the occasional vending machine treat delivered to them by Amy, the men experienced much more than just claustrophobia as they were forced to face their personal and professional insecurities.
Sometimes, it takes overcoming one small fear in order to unearth a much bigger one hiding underneath.