Tetrahedron in Bottrop

The Tetrahedron in Bottrop (German: Bottrop Tetraeder, or, officially, Haldenereignis Emscherblick) is a walkable steel structure in the form of a tetrahedron with a side length of sixty metres, resting on four nine-metre-tall concrete pillars. It is located in Bottrop, 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. The Tetrahedron is one of several Halden sculptures in the Ruhrgebiet region located atop slag piles, remnants of the region's industrial era. The design is reminiscent of the Sierpinski tetrix: placing four half-size tetrahedra corner to corner and adding an octahedron in the middle, a full-size tetrahedron is formed; this process can be repeated recursively to form larger and larger tetrahedra.