A sand sea, also called an erg, is a huge area of sand dunes.
The largest sand sea in the Western Hemisphere is in central Nebraska.
The Nebraska Sand Hills are a 20,000 square mile (52,000 sq km) dune field that covers the western third of the state.
Because the area receives adequate rainfall, grasses have covered the dunes, preventing them from eroding.
Thousands of years ago, sand from a dried inland seabed was blown into this area and formed the dunes.