Chelmno, Polish Chełmno, German Kulmhof, Nazi German extermination camp on the Ner River, a tributary of the Warta, in German-occupied western Poland.It opened in December 1941 and closed in January 1945 and was operated to execute Jews, most of whom were Polish. Some Soviet prisoners of war and more than 4,000 Roma (Gypsies) were also executed here.

: https://www.britannica.com/place/Chelmno-concentration-camp-Poland