CopenHill, Copenhagen

CopenHill (Amager Bakke) in Copenhagen is the world's cleanest waste-to-energy plant, outperforming #7 One Angel Square's waste-to-heat approach with its direct generation of electricity from non-recyclable waste. Designed by BIG and completed in 2019, the building's tilted roof features a 400-meter dry ski slope open year-round, alongside a climbing wall and hiking trail. Its chimney emits a visible smoke ring — formed of condensed water vapor — for every metric ton of CO2 emitted above a set target, making pollution transparent. The plant processes 440,000 tons of waste annually, generating enough electricity to power 150,000 homes, and is 20% more energy-efficient than the average European waste-to-energy facility.