It's really quite difficult to explain how one feels when walking around Auschwitz. There aren't really words that feel completely adequate in trying to describe it. The closest I've come to summing it up is by saying it's unbelievable, but even that is meant in so many ways. It's unbelievable in both a good and bad way.
The tour itself was very well done, both by our guide and by the way the museum operates the tours (you have audio sets and headphones that are set to the channel your guide speaks into, so you're listening to a live tour rather than a recording, and even though it's a group tour, it has an appropriately chilling effect of isolation as you listen to the guide via earphones). Auschwitz is also incredibly beautiful, which feels an odd thing to say, but it's true. The old buildings and the leaves that cover the entire spectrum of autumnal colours all lend to a pleasing aesthetic...
Until you walk into one of the blocks and are faced with photographs of starved children, or stopped in front of the execution wall, or peering into windowless cells measuring less than one square metre in which 4-5 people would have had to stand as punishment, or standing in a gas chamber where up to 700 people at a time were mass murdered and where the candles lit in memorium are electric, because there's so much gas seeped into the stone building that actual flame poses a fire hazard. That's where the bad unbelievable comes in - the sheer incomprehensibility of these events, even in the face of the evidence. Did you know they collected the hair from all the human corpses and used it to make netting and textiles? To sell? I didn't. And seeing the massive display of human hair and then the display of a woven cloth that has been scientifically examined and tested to conclude that it is in fact human hair... Incredibly disturbing.
The tours happen in two parts. The first is at Auschwitz I, where the labour camp was, and the first (and only undemolished) gas chamber remains. The second part is at Auschwitz II - Birkenau, where the camp prisoners resided, where people were first brought, sorted, and registered, and where the primary two crematoria one stood.
When I went, it was raining. Half of part one of the tour was outside, and the entire second half, at Birkenau, was outdoors. So we walked around the camp out in the rain for the day, and it was miserable. Especially part two - no umbrella or hooded garment, boots soaked through from numerous puddles and a vast amount of ashy (yes, those ashes) mud, socks and feet subsequently wet, jeans damp and stuck to legs, water endlessly tricking down the face. It was cold and wet and miserable. But honestly, I'm almost glad it was. Because when you're at Auschwitz and seeing the wooden-slatted "bunks" they slept crammed together in, with conditions being that of starvation and therefore dysentery so diarrhea was soaking into and seeping through the thin so-called mattresses onto the people in the bunk below, and the smell, and the absence of washrooms or toilets or any kind of drinkable water.... The misery of being out in cold rain for a bit is put into perspective. It's a freaking picnic on a warm summer day in comparison.
So yeah, Auschwitz was pretty fucking unbelievable. And that's really the only way I can think to put it.