In two years' time, a massive hole in the heart of Berlin will become the German capital's biggest reservoir, capturing an increasingly precious resource -- water. Once a swamp centuries ago, Berlin has gone from a city trying to keep its abundant groundwater from overflowing to one scrambling to keep its forests green. "The sponge city is also helping us to cope with climate change, with heavier rainfall and longer periods where it's dry. The sponge city helps us conserve rainwater on-site, making it available for vegetation," says Darla Nickel, director of the Berlin Rainwater Agency. IMAGES AND SOUNDBITES