Lovely. Agnes Denes' Tree Mountain: A Living Time Capsule, in Finland. Matt Olson at ROLU writes:
Eleven thousand trees were planted in a complex mathematical pattern by eleven thousand people from around the world, to be maintained for 400 years. One of the largest environmental reclamation sites in the world, Tree Mountain, created from refuse material from a gravel pit, was declared a national monument to serve future generations with a meaningful legacy. Dedicated in 1996 by the President of Finland, dignitaries, and participants from around the world.