 A global project to track the journeys of specific rain drops is harnessing the power of big data to monitor water supplies for our thirsty planet. Every month, for over 50 years, researchers have gathered rainwater from stations like this in Vienna and hundreds of other sites around the world. The unique isotopic signatures or fingerprints of each rain sample are recorded, and this information is entered into a large online database where it's compared to similar data from surface water to see which rain drops end up in which rivers. This large amount of data allows researchers to better understand the water cycle, for example, how, when and where water is recharged. This information is key for managing water resources, especially in the light of climate change. The datasets and maps are available to download for free and can be used in many ways. For example, in Costa Rica, where they're helping the government identify key locations to protect groundwater so they can target conservation measures and avoid drought. We now know which areas need special attention. We know how to protect them to ensure water supply for now and the coming decades.