German Unity Day
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 60m, resting on four 9m 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.