Riding a motorbike through the Tengger Caldera. In the distance, you can see Lautan Pasir (the "Sea of Sand".
This very soft sand is actually thin volcanic ash, and there is about 5 Km of it to cross.
For more information about the Bromo Tengger Semeru National Park, read en.wikipedia.org/wiki/Bromo_Tengger_Semeru_National_Park