From the World Heritage inscription:
The technological and other structures of the Zollverein XII Coal Mine Industrial Complex are representative of a crucial period in the development of traditional heavy industries in Europe, when sympathetic and positive use was made of architectural designs of outstanding quality. Zollverein is an exceptional industrial monument by virtue of the fact that its buildings are outstanding examples of the application of the design concepts of the Modern Movement in architecture in a wholly industrial context.
Consolidation of the Zollverein mining claim area was completed in December 1847, when it was the northernmost mine in the region. It belongs to the Gelsenkirchen anticline, in which the coal seams are deeply stratified. Mining began in the mid-19th century at a depth of some 120 m and finished at 1,200 m. By the end of mining the underground roadways extended over 120 km; they were accessed by 12 shafts, opened up progressively between 1847 and 1932. When Zollverein XII was opened, the earlier shafts were used solely for the movement of men and supplies; all the extracted coal was handled by the new shaft until the mine closed in 1986. The methods of mining evolved as technology developed from hand picks to mechanized coal cutting. The coals being extracted at Zollverein were especially suitable for coking. Consequently, the first stack-type coke-ovens were built there in 1857. The coking plant expanded considerably over the decades that followed.
Few people will travel to visit an abandoned industrial facility, yet I must confess that I enjoy visiting such sites almost more than I do visiting Roman ruins. Every industrial world heritage site I’ve visited has exceeded my expectations.
The Zollverein coal mine in Essen is a massive facility. Out of operation for more than 20 years now, the complex was a complete mining and coking facility which employed over 8,000 people at its peak. It was one of the largest coal mining facilities in the industrial Ruhr Valley.
Almost all of the buildings have been left intact even though most of them have found new uses as office spaces and museums. Even the museums have been build around the old machinery which remain in place where they once operated. One museum is dedicated to the history of the Ruhr Valley and another is the Red Dot Museum of contemporary design.
I recommend also visiting the coking plant which is the most industrial looking and larger of the facilities.
The grounds of the complex are open to the public for walking or biking. There are guided tours available in German and English which meet in the largest building up the extremely long escalator. Expect to spend anywhere from 1-3 hours at Zollverein, depending on the tour you take. There are many hidden nooks and rooms that can be seen on part of a tour that are not otherwise available to the public.
View my complete list of UNESCO World Heritage Sites.