Though the Nazis tried to keep operation of camps secret, the scale of the killing made this virtually impossible. Eyewitnesses brought reports of Nazi atrocities in Poland to the Allied governments, who were harshly criticized after the war for their failure to respond, or to publicize news of the mass slaughter. This lack of action was likely mostly due to the Allied focus on winning the war at hand, but was also a result of the general incomprehension with which news of the Holocaust was met and the denial and disbelief that such atrocities could be occurring on such a scale.
My late father was one of those who helped liberate the camp. He said the stench of the place went out 10 miles and was utterly revolting. Now you asked if there was a gas Chamber. There was a gas chamber on the site. It was underground with a ramp leading down to it from a wooden building above. Because the number of bodies was so great, the colonel made the snap decision to bulldoze the Gas chamber complex and use it as a huge grave pit. My Dad supervised the bulldozers that ripped off the roof and engineers blew up the interior walls. The place then became a grave pit for thousands of corpses. I believe there is a photographic record.
The camp was liberated on April 15, 1945, by the British 11th Armoured Division. The soldiers discovered approximately 60,000 prisoners inside, most of them half-starved and seriously ill, and another 13,000 corpses, including those of Anne and Margot Frank, lying around the camp unburied. The horrors of the camp, documented on film and in pictures, made the name "Belsen" emblematic of Nazi crimes in general for public opinion in many countries in the immediate post-1945 period. Today, there is a memorial with an exhibition hall at the site.