On a Sunday in May, people from all over Vienna gathered on Austria's largest landfill to enjoy music, candy floss, guided walks, and the chance to spot Hellgirl, one of the once-endangered Pinzgau goats that are its eco-friendly lawnmowers. The landfill supports biodiversity, helps endangered species rebound, and warms homes. Vienna, the world's only major city to treat and deposit residual waste entirely in city limits, was ranked Austria's cleanest region in 2017. Controlled burning in its four incinerators reduces waste to about 10% of original volume while providing heating and hot water for up to 400,000 households. Once metals are removed, the sand, water and cement are added to make terraces. Three meters of soil are then piled on top. The hill, 40 meters high, will grow another 35 meters by 2065. To cut household waste, the city offers free tours of its landfill and incineration plants, a second-hand store, hotline, and educational events for children.