Season 2, Episode 23 - The Monopolar Expedition
Aired May 11, 2009
When Sheldon is offered the opportunity to conduct research at the North Pole, he invites the rest of the guys along with him. The impending three-month expedition forces Penny & Leonard to reconsider their feelings for each other.
Season 6, Episode 13 - The Bakersfield Expedition
Aired January 10, 2013
The guys are attending Bakersfield Comic-Con dressed as "Star Trek: The Next Generation" characters, but after their trip goes off course they lose faith in their geeky interests. Meanwhile, the girls try to show more an interest in comic books.
Season 8, Episode 6 - The Expedition Approximation
Aired October 20, 2014
Sheldon and Raj consider applying for a dark matter research expedition in a salt mine, but their friends don't believe they would be able to cope with the difficult conditions. Sheldon and Raj test their hardiness to heat and cramped conditions in the university's steam tunnel.
Meanwhile, Leonard and Penny get into an argument over money. When they seek their married friends' help on how to handle their finances as a couple, it sparks a disagreement between Howard and Bernadette.