When it has Arbeit Macht Frei written above the entrance and harbours the apparatus for millions of executions. Auschwitz has been open to the public for a good number of years. Do you need to justify a visit? I guess some tourists have a conscience-wrestling match and decide to skip it for various reasons, but judging by the coach park and the organised tours from Krakow, curiosity gets the better of most.

I saw many older people crying and also busloads of schoolkids play-fighting, bored. What can it mean to a ten year old? How do you begin to explain it? Concentration camps are a part of history, albeit history in its most awful guise. This is no First World War battleground, where your imagination has to add noise and mud and gunfire. All the fixtures are still at the camp, the barbed wire, the ‘showers’, endless railtrack.

A short film forms an introduction to the horrors. Full of crackly edits and stomping boots. It looks so cold in black and white and the striped prisoners all drained and gaunt. Leaving through an unremarkable door, you walk into the camp with its lying sign suggesting work brings freedom.

It’s the scale that hits hardest. The order and symmetry of the construction is terrifyingly vast. Whole rooms contain the remains of the everyday; shaving brushes, shoes, hair, spectacles. All stacked to the ceiling and stripped of their context. The photographs are equally harrowing; bodies stacked like butchers meat, the shaved heads and twiggy limbs. One of an uncovered mass grave was grotesque and simultaneously compelling. One corpse was lying half submerged by the hardened mud and you couldn’t work out where the body ended and the ground began.

The showers were simply squat brick buildings. The sort of structure that graces any campsite. Except, in camp, you have options. These were one-way showers, a mass guillotine. You can view the holes where Zyklon B was piped in. I remember a tourist asking the guide a very detailed scientific question about the composition of the poison. The guide became impatient, “does it really matter what it was?”

On the way out, a young boy asked a received wisdom question, “is it true that birds don’t fly over Auschwitz?” The guide had a stock response, “birds fly over the camp, but they never sing.” The rain started hammering down as I came out. I rushed to find cover. By the exit is a café, but how can you eat in Auschwitz?