CopenHill, Copenhagen

BIG's CopenHill (Amager Bakke) in Copenhagen (2019) is the world's cleanest waste-to-energy plant and also a ski slope, hiking trail, and climbing wall. The building's tilted roof is covered in a dry ski slope accessible year-round, with a wall of artificial ski jumps on its north side. Its chimney emits smoke rings when CO2 levels exceed targets — an architectural provocation making pollution visible.