According to good ol' Wikipedia...
The Tottori Sand Dunes (鳥取砂丘) are unique sand dunes located near Tottori City in Tottori Prefecture, Honshū, Japan. They are the only large dune system (over 30 km²) in Japan.
The dunes were created by sediment deposits carried from the Chūgoku Mountains by the Sendai River into the Sea of Japan. Sea currents and wind help bring the sand from the bottom up onto the shore, where the wind constantly rearranges their shape. The dunes have existed for over 100,000 years, but the area of the dunes has been steadily decreasing due to a government reforestation program following World War II.
Each year, around two million visitors—mostly from within Japan and East Asia—visit the dunes.
Fortunately, since we owned a car, we made the drive there ourselves and arrived in under two hours. Because it was a public holiday (Children's Day), the area was quite busy, but we didn't let that stop us. There's a fair amount of things to see/do there: a museum (which we didn't bother with, but in photos it does look cool), a snack area, camel rides, paragliding, and of course, the sand dunes themselves that overlook the Sea of Japan.