The Zollverein XII pit is a unique monument to the architectural and engineering prowess of the Ruhr area’s coal industry symbolised by the double-rack winding tower of pit XII. Built between 1928 and 1932, Zollverein was regarded as "the most beautiful coal mine in the world". At the same time, it used to be the world’s biggest and most modern colliery, producing 12,000 tonnes of hard coal a day. It was closed in 1986. The surface installations around pit XII were designed by the architects Schupp and Kremmer, who created a symbiosis of form and function. Since the 1990s, the Zollverein site has been developed according to the master plan of Rem Koolhaas into a design and culture venue staging top-class cultural events. In 2001, the whole complex was accorded the status of a UNESCO World Heritage Site.