The Vertical Earth Kilometer by Walter De Maria is one of the most famous and least visible artworks in the world. Installed for documenta 6 in 1977, it is a solid brass rod two inches thick and one kilometre long, drilled straight into the bedrock beneath the Friedrichsplatz. The only thing on the surface is a flush sandstone slab with a small brass disc at its centre.
It is, by design, an act of pure faith — the visible artwork is the knowledge of what is below. The work sits in the middle of the square in front of the Fridericianum; you can stand on it and weigh your relationship to gravity, geological time, and a piece of art you cannot, by definition, ever fully see.