The Groene Kathedraal is a piece of land art by the Dutch conceptual artist Marinus Boezem, hidden in a patch of polder farmland in the Oosterwold area on the eastern edge of Almere. He planted 178 Lombardy poplars in 1987 in the exact ground plan of Notre-Dame de Reims — towers, nave, transepts, choir — so that the trees rise where the cathedral's walls and columns would have stood. Walk between them and you find yourself in a perfectly proportioned Gothic interior made of bark, leaves and sky.
Next to it Boezem laid out a paved counter-piece, the Cathedral of Light: a stone outline of the same plan, marked in the grass, with a vanished cathedral where the real one's shadow would fall. The site is free, never crowded, and reached by a small farm track off the Vogelweg. Bring a bike — there is no parking nearby.