Auschwitz was established by the Germans within the area of occupied Oświęcim as a camp for Polish political prisoners. From 1942, Auschwitz-Birkenau soon became one of the main centres for the mass extermination of European Jews. In Auschwitz, the Germans killed at least 1.1 million people, mainly Jews, but also Poles, Roma, Soviet prisoners of war and people of other nationalities. The Auschwitz-Birkenau State Museum was established on 2 July 1947 on the area of the former concentration camp. The Memorial Site covers the area of the two extant parts of Auschwitz I and Auschwitz II-Birkenau. This makes up nearly 200 acres of land, 155 buildings and 300 ruins (including the ruins of gas chambers and the crematoria) and more than 100 thousand of personal belongings of the murdered, other items, archival documents and prisoner works of art. Maintaining the authenticity of the Memorial Site are care specialists from various areas of preservation, who work in modern conservation labs. It is the most visited museum in Poland. In 2012, the Auschwitz Memorial Site was visited by 1.43 million people from around the world. The vast majority of visitors explore the areas of both parts of the camp, Auschwitz I and Auschwitz II-Birkenau, under the care of nearly 300 Museum educators guiding in almost 20 languages. In 1979, the area of the former KL Auschwitz-Birkenau was entered into the List of the UNESCO World Heritage Sites, and the date of the liberation of Auschwitz – 27 January – was adopted by the United Nations as the International Day of Remembrance for Victims of the Holocaust. Museum is a scientific research and education institution. It collects, develops, maintains and makes available documents and former camp items. From 2005, the Museum has opened the International Centre for Education about Auschwitz and the Holocaust.