Esterwegen opened in the summer of 1933, among the very first camps the new regime built, designed to hold two thousand men classified as political Schutzhäftlinge - "protective custody" prisoners, an official phrase meaning they had been imprisoned without trial for opposing the government. Communists, Social Democrats, trade unionists, journalists, lawyers. For a period it was the second-largest concentration camp in Germany, after Dachau.

The prisoners had their own name for it: Hölle am Waldesrand, hell at the forest's edge. Esterwegen was one node in what grew into fifteen camps scattered across the Emsland and the County of Bentheim between 1933 and 1945. Around seventy thousand camp and penal prisoners passed through that network, along with more than a hundred thousand prisoners of war.

The official justification for the Emsland camps was land reclamation. The bogs of north-western Germany were, in the regime's telling, a resource waiting to be drained, cut and farmed, and the men in the camps were the equipment. Peat had real economic value, and the reclaimed ground would become fields.

But the labour served a second purpose that nobody troubled to disguise. Peat is worked wet. Men stood in standing water, cut and stacked blocks by hand, and dug the drainage channels that would eventually carry the water off - all day, in all weather, under guards who answered to no law that could be invoked against them. The point was exhaustion as much as output. The song's title is not a metaphor: the men who wrote it were describing their actual working day, and they gave themselves a soldier's name for it because it was the only dignity within reach.

One man held at Esterwegen is known to almost everyone who studies the period, because a photograph of him survives from 1934. Carl von Ossietzky was a pacifist journalist and editor of the weekly Die Weltbühne who had already served time for publishing evidence that Germany was secretly rearming in breach of the Versailles Treaty. He was arrested after the Reichstag fire in 1933 and ended up in the Emsland.

In the photograph he is gaunt, in camp clothing, looking directly at the lens. In 1936 the Norwegian Nobel Committee awarded him the Peace Prize that had been held over from 1935. The German government refused to let him travel to Oslo to collect it. He had contracted tuberculosis in the camps, and he died of it in a Berlin hospital on 4 May 1938, aged forty-eight, still under guard. The cabaret comedian Werner Finck, whose jokes had cut too close, spent six weeks at Esterwegen and got out.

The concentration camp closed in the summer of 1936, but Esterwegen kept working. Until 1945 it ran as a prison camp, holding political prisoners and, later, men taken under the Nacht und Nebel decree - people seized in occupied Western Europe and made to disappear deliberately, with no word sent to their families about where they were or whether they were alive.

The staff simply moved through the system. SS-Hauptscharführer Gustav Sorge, a guard here whom his own colleagues nicknamed "the Iron Gustav" for his brutality, was transferred to Sachsenhausen and was convicted of war crimes after the war. When the fighting ended the British used Esterwegen as an internment camp; then it became a prison; and then, until 2000, a depot for the German Army.

For decades the Emsland camps went largely uncommemorated. A documentation centre operated in nearby Papenburg from 1985, driven mostly by local people who believed the record ought to exist somewhere. Not until 31 October 2011 did a full memorial open on the site itself. The Gedenkstätte Esterwegen now serves as the central place of remembrance for all fifteen camps.

It reconstructs nothing. Where the barracks stood there are marked outlines and low planting; from the position of the former front gate you look straight down the line of the old camp road. One path leads out of the camp and toward the bog - the way the men walked every morning with their spades over their shoulders. It is a short walk. That is part of what the place is telling you.