In the beginning of the 20th century, Maastricht was an industrial city, with many factory's. Jan Pieter Minckeleers - a scientist from Maastricht - was the first to discover Coal Gas, making the first city lights available. In 1940, Maastricht was conquered by the German forces. However, many paintings were hidden in the caves at Sint Pietersberg - a steep hill in the south-west of Maastricht. Even De Nachtwacht was hidden there. Maastricht was liberated on 14 September 1944. In 1992 Maastricht was the host-city for the 1992 European Treaty, where the EU was officially formed and a decision was made about the Euro.