Space launch mountain
Use the ever expanding Sahara desert to construct a 13.5 mile tall pyramid of sand composite with a 1/2 square mile launch pad at the top. It would require the movement of around 3,200 cubic miles of sand that the 3.5 million square mile desert could easily supply. After initial construction costs for space launch would become minimal and could be used for centuries to come.
Reward: Human enlightenment