With an output of more than 23,000 tonnes of raw coal per day, the Kokerei Zollverein (Zollverein colliery) was once the most efficient colliery in the world. The central shaft XII, built between 1928 and 1932 according to plans by Fritz Schupp and Martin Kremmer, is considered a modern technical and aesthetic masterpiece. In 2001, the site was officially listed as Zollverein Coal Mine Industrial Complex on the UNESCO World Heritage List. The Kokerei Zollverein (Zollverein coking plant) was built between 1957 and 1961 in connection with the Zollverein colliery. Approximately 7,500 tonnes of coke could be produced here daily from around 10,000 tonnes of coal. The Salzlager (salt warehouse) where fertiliser – made from ammonia and sulphuric acid – was produced until the 1980s is located on this site. The coking plant was shut down in 1993. Since 2001, it has housed the walk-in installation Palace of Projects by the artist couple Ilya and Emilia Kabakov.