In view of extreme water shortages in major coastal cities — last year’s in Cape Town, South Africa, and this year’s in Chennai, India, are just two examples — we argue for the investigation of offshore groundwater reservoirs as a potential solution.

These reservoirs are found in marine sediments and developed when large continental shelf areas became exposed during the last Ice Age.

We evaluated the distribution of these reservoirs in relation to the likelihood of water shortages in nearby coastal cities (see go.nature.com/2mkmyat). Cities around the East China Sea have many potential sites, as do the Sunda Shelf and the Australian coast. Some of the reservoirs with greatest potential are located conveniently for Jakarta, Cape Town, Singapore and Beijing. However, there is limited potential in Chennai.

There is an urgent need to map these resources, determine their likely responses to climate change and assess how much water they could sustainably supply.