A 1,400-year-old Peruvian method of diverting water could supply up to 40,000 Olympic-size swimming pools' worth of water to Lima each year. That information comes from a new study published in Nature Sustainability. It's one example of how ancient methods could support existing modern ones in countries without enough water. More than a billion people across the world face water shortages. Man-made reservoirs store rainwater and water overflow for use during drier times. But reservoirs are costly, require years to plan and can still fail to meet water needs. Recently, for example, the reservoirs in Chennai, India, went almost dry. The city’s four million people had to then depend on water transports from the government. Peru's capital, Lima, depends on water from rivers high in the Andes Mountains. It takes only a few days for water to flow down to the city. So when the dry season begins in the mountains, the water supply quickly disappears. The city suffers shortages of 43 million cubic meters during the dry season. It resolves this with modern structures such as man-made reservoirs.