In 1940, Witold Pilecki told his superiors of his plan to enter Auschwitz, gather intelligence, and organize enough inmate resistance to escape. At the time, very little was known about the extent of what the Germans were doing in the camps. 

It was generally thought they were internment camps, not death camps. The plan was approved, and in September 1940 he went out during a Warsaw street roundup and was brought to Auschwitz after two days of beatings. While there, he gathered intel and organized an underground union. 

The tasks of the union were mainly to provide news from the outside, improve morale, give extra food and clothing, and to train the inmates to take over the camp if Allies attacked it. The union also provided Poland with information on the camp, and in 1941, it was even being broadcast on a radio transmitter built by the inmates. 

Ultimately, Pilecki’s information did help Poland learn much of the atrocities of the Nazis and eventually liberate Auschwitz, but was unable to gain immediate help from the Allies because the British thought the stories were greatly exaggerated. 

(Source)