Remote wilderness.
At the northern boundary of Death Valley National Park in California, the mountains of the Last Chance Range ring the towering Eureka Sand Dunes, the tallest dunes in the park. Another elusive system of dunes is located in the area, the appropriately labeled Hidden Dunes. Eureka Valley is reached via a long drive on a rough gravel route.
[Update: It was reported today (08/27/2014) that one of Death Valley’s mysteries, that of the moving stones at Racetrack Playa, has been solved. See this and this.]