In the months that followed, over 200,000 Jews, Soviet prisoners of war and Roma were murdered here, making it the fifth most deadly extermination camp.
Yet it receives few visitors. Today, the distant sound of the traffic on the main Berlin-Warsaw highway — originally built by built by slave labourers — rises from the valley below, but is easily drowned out by mooing cows in a barn next to the deserted car park.
Bartlomiej Grzanka heads up the four-man team that runs the Kulmhof museum, which first opened in 1990.
“A monument was put up in the forest in the sixties, when I was born, so I grew up knowing what happened here,” he said.
“Chelmno was the first death camp and here the Nazis experimented with ways to carry out mass murder and dispose of the bodies.”
More and more local schools have brought children on day trips to the camp, which has prompted investment in the new museum. Yet Kulmhof still had fewer than 15,000 visitors last year. By contrast, over two million people visited Auschwitz.
Mr Grzanka hopes that the new museum will change things, but adds: “Auschwitz is a symbol, as it had a lot more survivors and the victims came from all over Europe. Chelmno was a local camp and its victims came from nearby.”
Like the others who work at Kulmhof, Adam Dominiak, 27, was raised locally. His grandfather was a slave labourer on a farm taken over by German settlers.
Operations at Chelmno were top secret and, when the locals passed by the death pits in the forest, they were ordered to look straight ahead or they would be shot, he says. Even the postman who brought letters to the camp had to stand with his back to it.
But the stench of rotting bodies could be smelt for miles around so the Nazis began to burn them. Standing in the forest clearing where they built the first crematoria, Mr Dominiak says that “the official story was that the Germans were baking bread in the middle of the forest.”
He adds the macabre fact that when the Germans noticed the grass grew greener where the ashes were scattered, “the camp started a side-line business selling bonemeal to Polish farmers, who scattered it on their land thinking it was from an abattoir.”
Many of the victims had travelled the 50 kilometres from the ghetto in Lodz, where Leib and Stenia Sniatkiewicz, the children of Holocaust survivors, live today.
They make a point of attending events at Chelmno and meeting schoolchildren, especially during Chanukah, but they are now in their seventies and with few remaining Jews in Lodz they are pessimistic about what will actually be remembered when they are gone.
A controversial law passed by the Poland’s parliament earlier this year made it an imprisonable offence to suggest the Polish nation or state was complicit in the Holocaust. But given how hot a topic it has been, it is surprising how few people know about the camp at Chelmno.
Local resident Iwona Kalkowska, 40, has been animated by Jewish culture all her life and scoured her family tree in the hope of finding a Jewish relative. Despite her extensive research, she had never heard of the camp until now.
“I can’t tell you how shocked I am,” she says. “I have lived here all my life but never knew it existed.”
Ms Kalkowska complains that her parents have never answered her questions about the past. “They let out tiny snippets sometimes like trailers for a huge TV drama but clam up the moment you ask a question. At school nobody told us anything.”
It is for this reason Mr Grzanka thinks the new museum will have “an important educational role”.
“It is crucial that people know the history of this place so that it doesn’t happen again,” he says.
“We have a database of 40,000 names that we will draw on to tell the story of the individuals who died here. Personal stories matter as numbers do not mean much to people.”
