For several decades, the Sobibór memorial was a simple collection of monuments and a tiny museum. It was moving in its simplicity, the paucity of visitors adding to the contemplative atmosphere. The site is now being massively transformed. A new museum is set to open by 2021, with exhibits designed for visitors several generations removed from WWII. In the meantime, much of the memorial is technically closed, but you can arrange visits in advance and there is a public viewing area.
Sobibór was one of three extermination camps built by the Nazis (along with Bełżec and Treblinka) as part of Operation Reinhard, the plan to kill all the Jews living in occupied Poland. The Nazis picked this remote swampy site to escape notice. The camp was only 24 hectares and the staff of Nazi and Ukrainian guards never numbered more than 150. The train tracks where boxcars filled with Jews arrived are still in place. Victims were ordered out of trains, stripped of their clothes and belongings and separated from loved ones. Women had their hair shorn off so it could be used in the German war effort (mattress stuffing).
Amid shouts and beatings, hundreds of people at a time were then force-marched up the cynically named Road to Heaven (Himmelfahrtstrasse) and into the gas chambers. Few people who arrived at Sobibór lived for more than hour. Jews from elsewhere in Europe were also killed here. From the Netherlands, over 34,000 Dutch Jews, distributed over 19 trains, were brought here. Of these, only 18 survived and returned to Holland after WWII.
On 14 October 1943, the camp's slave workers revolted. Led by a Soviet POW, they killed German and Ukrainian guards and some 300 escaped (an event recalled in several movies, including the 2018 Russian film Sobibór). Ultimately only 50 survived the intense efforts to hunt them down. In response, the Germans shut down Sobibór. The site was plowed under and trees planted to try to hide the evidence. Despite there being fields of bone fragments and ashes, Sobibor's isolation helped it slip into obscurity; the first memorial wasn't erected until 1965.
Sobibór is now a branch of the Majdanek State Museum. At the end of 2008, the governments of Poland, The Netherlands, Slovakia and Israel agreed to significantly upgrade the facilities. Since then the project has proceeded with the sort of fits and start not unexpected with such a diverse group of stakeholders. The open ground which was covered in human ashes and bone fragments has now been covered in bright white stones. The new museum is planned to match the quality of the facilities at Bełżec and Madjanek. Contact the memorial staff to get permission to enter the compound. You can also view the main site from a public footpath behind a chapel 150m north of the new museum site.