Marshall Islands

In the Marshall Islands, fresh groundwater can sit inside the sand, floating above saltwater.

The islands are low and made largely of porous coral sand and gravel. When rain falls, water seeps down between the grains.

Fresh water is less dense than salt water, so underground it can collect as a lens floating above the saline groundwater.

Researchers measured one such freshwater lens on Kwajalein at almost twelve metres thick.

It grows with rainfall and shrinks during drought and heavy pumping. The sand forms both the ground above and the porous reservoir around the water below.