CNN - A 7,700-kilometer wall of trees, running through 11 countries along the southern frontier of the Sahara Desert.

That's what the African Union proposed in 2007, a "Great Green Wall" that was to be the largest living structure on the planet.

The purpose was to provide a mighty barrier against the advance of the Sahara, and to reverse the plague of desertification spreading drought, famine and poverty through the Sahel region.