While the pointed hill in Ylöjärvi, Finland known as Tree Mountain looks like an oddly symmetrical natural forest, it is actually a colossal work of art that stands as the world’s first virgin forest created by humans.

Proposed in 1982, the hill itself had to be built before the spiraling rows of trees could be planted. Once the land itself was built, the artist, Agnes Denes, enlisted 11,000 people from all across the globe to plant 11,000 trees in a specifically designed pattern. The pattern was designed to evoke ancient earthworks as well as the mathematical precision found in many of the works of the painting masters.

The planting took place over four years from 1992 to 1996. With everything set to roll, the artist then let nature do its thing. The trees flourished on the reclaimed land, and by the early 2000s the intricate pattern was beginning to show as though it had occurred naturally.

Announced at the 1992 Earth Summit, the environmental art project is intended to help alleviate the world’s ecological stress. The Finnish government has pledged to protect and maintain it for 400 years. The forest has continued to thrive over the decades, and is now so lush that the original pattern can be hard to perceive.