We recently went to Bermuda on vacation, and flying in we noticed the limed, stepped roofs of all the houses. We thought it was simply beachy aesthetics, that is, until our taxi driver explained the real, eco-friendly reason why....
All the houses on the island have treated roofs to catch rainwater. The stepped roofs help with runoff, and water is stored in cisterns underground where it's purified and filtered.
The roofs need to be limed every two years or so. Almost every single home and residence captures rainwater, though some of it's supplemented with municipal water. Being such a tiny island, they've had to figure things out with limited resources (there are also no fast food chains and tourists cannot rent cars). It was such a great lesson about conservation!