The Tottori Sand 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 highest dunes reach around 90 meters above sea level and can reach 40 degree slopes. Technically, its not a desert but the temperature of the sand can easily soar above 50C on sunny summer days. A number of hardy species of plants and animals also thrive in sections of the dunes.
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. Additionally, concrete barriers erected to protect the coast from tsunamis have disrupted the currents responsible for bringing the sand to shore.
The Tottori Sand Dunes attract some two million visitors each year, mostly from within Japan and East Asia.
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Sources: 1, 2, 3