This is the most difficult thing I’ve ever written.

The original camp at Auschwitz, later known as Auschwitz I, consisted primarily of a converted Polish army barracks. It was soon judged to be too small—there were so many people to kill. A series of other satellite camps was therefore built. By far the biggest of these was Auschwitz II, Birkenau. Both of these camps can be visited. Both should be visited.

Auschwitz I is entered through the famous iron gate promising that work can make you free. If you didn’t know what the camp had been and you didn’t enter any of the buildings, then it could even pass for pleasant, aside from the electrified barbed wire surrounding it. Two-story brick buildings are arranged in neat rows and there are trees lining the pathways. But you do know what it was, and you will enter the buildings.

There is a room filled with hair. Two tons of shorn women’s hair, some still braided. It was used to make fabric. The hair you can see is just what was left over when the camp was abandoned before being liberated. The room smells of hair. Another room holds thousands of pairs of eyeglasses. Another has prosthetic limbs, trusses, and other personal medical devices. Another holds children’s clothing. There are very small shoes there, and gaily-colored socks. There’s a broken doll in a display case. Yet another room holds mountains of adult shoes: elegant men’s shoes, work shoes, sandals, wooden clogs, high heels.

Auschwitz I is well restored, easily accessed, well explained. It is surprisingly small. Auschwitz II, which is three kilometers down the road, is a vast, windswept plain. It is here that the trains arrived directly, passing through an arch, to unload on a long siding, where the passengers were immediately sorted into two groups: those who were fit to work and those who were not. Anything you’ve ever read by an Auschwitz survivor was written by one of the 20 percent or so who were deemed fit to work, because the rest were immediately gassed. Those who were fit to work died a little later, once they had been worked to death: an infinitesimally small number survived to write about it. The gas chambers are at the end of the railroad track. The Nazis blew them up just before abandoning the camp, but the ruins are there, and you can see where the people entered, where they undressed, where they were gassed, and where their bodies were cremated. It works like that, in one efficient line. There were five crematoriums. Often, these were insufficient and the excess bodies were burned in the fields behind them.

It is Auschwitz II that communicates the weight of Auschwitz, that demonstrates the sheer scale of slaughter that happened there and the scientific way in which it was applied. The camp is enormous, over 400 acres. Most of the huts were destroyed in 1945 in an attempt to hide it all—as if something like this could be hidden—but many remain. Unlike at Auschwitz I, where most of the prisoners’ quarters have been transformed into museums, here they were left as they were.

You can’t imagine this. Really, you can’t imagine it. You can, and you should, read the books written by the survivors, and you should see the movies about it, but you do have to come here. You must come here—I’ve written about all these “places you should go,” but this, sadly, is the one place you really must go above all others, because you’ll have to face the barren fact that this all happened, that human beings did this to other human beings. We are members of a species that is capable of this. At Auschwitz I, many of the buildings have rows of identity photos lining the walls, prisoners staring at you as you go by. Look closely and you’ll see yourself, in one form or another. At Auschwitz II, Birkenau, the individual fades away into the enormity of the numbers killed. Here you are standing in the middle of the world’s largest graveyard, in the center of the greatest evil ever conceived by man. When man applies science to evil, the scale of it is crushing.

I wandered around the abandoned buildings, alone, entered some of them, saw the small sleeping platforms, stacked three high, each of which held four or five prisoners. I walked farther along the grass and found the latrines, where there are endless holes in a long concrete slab covering what used to be a pit of filth. There is still German writing on the wall warning the inmates to be quiet. You can wander around the remains of the gas chambers and look at the fields behind them where the ashes were dumped. Near one field, there are reproductions of the only photographs existing that were taken by an inmate. They are a series of three pictures, the first showing naked Jewish women being driven into Gas Chamber V, and the next two showing piles of bodies being burned in the open air behind the gas chamber. I suppose the ovens were overworked that day. The pictures are displayed in front of the field on which the bodies were burned. It looks the same; the ashes of those in the pictures are there, under the grass.

At Auschwitz I, you can enter Cell Block 11. This was the prison within the prison, where those who were to receive special attention were confined, usually to die of starvation or suffocation, though often they were simply shot. You can see the tiny, stifling cells where they were held, including four “standing” cells, which are each about the size of a refrigerator, with only a small hole for ventilation, where up to four prisoners were detained for one or several days. Some of the thick wooden doors to these cells bear the marks of those who scratched at them, trying to widen the peepholes. Those few who survived their detainment in the cell block were forced to strip naked and then were taken outside and lined up against the wall in the courtyard between Cell Blocks 10 and 11, where they were shot. Several thousand people died in that courtyard. It’s not very big.

The crematorium at Auschwitz I remains, and it can be visited. I visited it. The first room is where the condemned stripped. The second room is the gas chamber. It is concrete. There are holes in the roof where the gas was introduced. There is a wreath of flowers on the floor, and some visitors throw roses onto it. There were Japanese tourists there that day; they pressed their hands together and bowed, with their eyes tightly shut. There was a woman who paused and cried out, as if shot, then left the place in tears. And there was me.