DE-4279066 DE-5316384 A German Landmark : The Tetrahedron in Bottrop (Bottrop Tetraeder)

At one time, this region of Germany was well-known for coal production.  These days, all the coal mines in Bottrop are gone.  To remember the hey-day of coal production, this structure called the Tetrahedron was erected.
The Tetrahedron in Bottrop (Bottrop Tetraeder or officially Haldenereignis Emscherblick) is a walkable steel structure in the form of a tetrahedron with a side length of 60m, resting on four 9m tall concrete pillars. It is located in Bottrop, North Rhine-Westphalia, Germany, on top of the mine dump Halde Beckstraße and serves as the town's landmark. It was opened on German Unity Day 3 October 1995.

I have received two postcards of this interesting structure.  The first postcard came from Donela (18 June 2015) Postcrossing and the second from Anne (13 July 2016) Postcrossing.