Operation Gunnerside took place in February 1942 in the small town of German-occupied Vemork, Norway. Nine Norwegian men specially trained by the British Special Operations parachuted just outside of the town with the objective of blowing up a heavy water plant.
This was considered important because heavy water was critical for the Germans to produce plutonium, is a crucial elect in noted development of a nuclear weapon. German scientists were rushing to build such a weapon before the Allies could.
The heavy water plant at Vemork was very fortified and could not be taken out by bombing, so sending in a small team to destroy it was essentially the only option. The nine men, who were lead by a twenty three year old Norwegian soldier named Joachim Ronneborg, had to scale a five hundred foot cliff in the middle of the frigid winter and then infiltrate a heavily defended basement laboratory.
The team succeeded and planted explosives that blew up part of the base, and effectively halted all German plans to build any kind of a nuclear weapon.