Across dozens of Chinese cities, roads, parks and neighbourhoods have quietly been redesigned over the past decade to behave less like solid concrete and more like something you could actually wring out. The idea is known as the sponge city concept, an approach to urban planning that swaps traditional drainage systems built purely to channel rainwater away as fast as possible for a softer, more absorbent kind of infrastructure designed to soak water up, store it, and release it slowly over time. China's president Xi Jinping first put the idea forward as national policy in 2013, and since then it...