Sobibor was a
Nazi extermination camp that was part of
Operation Reinhard.
Jews,
Soviet POWs, and possibly
Gypsies were transported to Sobibor by rail, and suffocated in
gas chambers that were fed with the exhaust of a diesel motor. About 260,000 people were killed in Sobibor.
http://auschwitz.dk/Sorbibor.htm contains a description of Sobibor.