The Great Sand Dunes form a sea of dunes covering many square miles, with the visible dunes as only the front lines. The dunes stretch 8 miles across, enough that a strong hiker would need 5 hours to cross them. The tallest dune rises nearly 700 feet, taller than a 50 story building. The amount of sand is immense: loaded into railroad cars, it would make a train long enough to circle the globe more than 20 times, and spread ankle-deep, it would cover 1/10 of Colorado. The grains are tiny, about 1 millimeter in diameter, though most measure between 1/8 and 1/4 millimeter, while smaller particles blow away over the mountains.