Sand on some beaches and dunes emits a distinct 'squeaking' sound when moved under pressure - e.g. by walking on it Videos here

It only affects certain types of sand - more rounded and drier grains generally don't squeak (as much). The underlying acoustic mechanism is as yet undiscovered.

More information from ABC ,'Catalyst', Australia, [archived]

"We can land a remote controlled robot on Mars but we don’t know why sand squeaks here on Earth and to be honest I kinda like that."

[ Ruben Meerman ]

A list of locations where squeaking sands can be found is maintained by the Acoustic Engineering dept. at the University of Salford, UK.

Also see :Sand dune formationplugin-autotooltip__plain plugin-autotooltip_bigSand dune formation

"Large swaths of Earth’s surface are covered in loose sediment. The grains that make up this sediment form fascinating bedforms from meandering riverbeds to wavy dunes, whose shapes are constantly changing as water or air currents move the grains.