For years, the 250 families of the 33-storeyed Avalon building at Powai continued to pay Rs 30,000 a month for tanker water. And then just before the rains this year, they invested in the credest form of rainwater harvesting - connecting open pipes from the top of the building to an underground tank. Now, they'll save Rs 1 lakh every monsoon. The Avalon experiment is proof of the vast potential offered by rainwater harvesting in a city reeling under water shortage.