Beneath our feet there is an invisible ocean. Within the cracks of rock slabs, sand and soil, this water sinks, swells and flows — sometimes just a few feet under the surface, sometimes 30,000 feet below. This system of groundwater provides a vital supply for drinking and irrigation, and feeds into rivers, lakes, and wetlands. Across the globe, it contains 100 times as much fresh water than all the world’s rivers and lakes combined.