For half the world's population, the water in their drinking glasses comes from below them. Groundwater also supplies 40% of global irrigation projects. Alarmingly, more than a third of the planet's aquifers, or groundwater basins, are dropping. Declining water tables leave entire regions vulnerable to drought, land subsidence or seawater intrusion while damaging ecosystems and reducing water access. Properly securing this resource is a matter of social, humanitarian and environmental security.