Sand Mountain is a large singing sand dune in western Nevada, about 20 miles east of Fallon on U.S. Route 50. Stretching roughly two miles and rising about 600 feet within a broader 3.25-square-mile sand field, it was formed from sand left by ancient Lake Lahontan after the lake largely dried up around 9,000 years ago. Its moving sand can produce sound under the right conditions, giving it a distinctive character. Protected today as the Sand Mountain Recreation Area, it supports off-highway vehicle use, sandboarding, and sand skiing, contains the ruins of Pony Express-era Sand Springs Station, and is the only home of the critically imperiled Sand Mountain blue butterfly.