Sobibór was a Nazi German extermination camp located on the outskirts of the Sobibór village, which was located in the eastern part of Lublin Voivodeship, Poland, close to the Chelm-Wlodawa railway line. The camp was 5km away from the Bug River, which today forms the border between Poland and the Ukraine.
In 1942 the area around Sobibór was swampy, densely wooded and sparsely populated. It was the second death camp to be constructed as part of the Aktion Reinhard programme, and was built on similar lines to Belzec, incorporating lessons learned from construction of that first death camp.
Jews from Poland, France, Germany, the Netherlands, and Czechoslovakia, as well as Soviet prisoners of war (POWs) (many of them Jewish), were transported to Sobibór by rail, and suffocated in gas chambers that were fed with the exhaust of a petrol engine.
One source states that up to 200,000 people were killed at Sobibór. Thomas Blatt claims that "In The Hague court proceedings against former Sobibór Nazis, Professor Wolfgang Scheffler, who served as an expert, estimated the total figure of murdered Jews at a minimum of 250,000."
See also Holocaust Research Project: Sobibór.
In the month of May 1942 the largest number of Jews were gassed, more than thirty-six thousand were brought to Sobibór from nineteen communities between the Vistula and the Bug.
After a successful revolt on October 14, 1943, about half of the 600 prisoners in Sobibór escaped; although most were later re-captured and killed, the camp was closed, bulldozed, and planted-over with pine trees days afterward to conceal its location. A memorial and museum are at the site today.
An article about Operation Reinhard in Swedish Wikipedia. The construction of Sobibór, Treblinka and Belzec exterminations camps with further links to other languages.