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On the patch of ground in Friedrichsplatz Park, within the German city of Kassel, is an unassuming gold circle. Measuring 2 inches in diameter and lying flush with the pavement, it looks like a coin that’s been dropped, forgotten, and trodden on over several decades. But this is no coin—it’s the top of a brass cylindrical rod that extends 1 kilometer into the ground in the name of art.

The Vertical Earth Kilometer is a permanent Earth sculpture, installed by Walter De Maria in 1977. An artist with a penchant for minimalism and land art, De Maria is also known for The New York Earth Room (a SoHo studio packed 22 inches high with dirt) and The Lightning Field, an expansive grid of silver poles in the New Mexican desert. The Friedrichsplatz Park installation is a companion piece to The Broken Kilometer (1979), for which De Maria took an identical kilometer-long brass rod, divided it into 500 equal segments, and placed the sections in neat piles in a New York gallery. It is still on view.

With nothing to mark its significance beyond a surrounding sandstone square, The Vertical Earth Kilometer offers a subtle message to passersby: Question what you see, for much may lie beneath.

Visit Atlas Obscura for more on The Vertical Earth Kilometer.

More incredible places around the world:

• A five-story, blood-red waterfall spills down Taylor Glacier in Antarctica
• Cameroon’s Lake Nyos once killed over 1,700 people in one night
• Beneath Odessa is a seemingly endless labyrinth of pitch-dark catacombs