Adrar Madet Massif, Niger, 1997

Photograph by George Steinmetz

"Stark circle of rock measuring about 60 feet [18 meters] in diameter lies in the Ténéré desert below the massif of Adrar Madet in Niger. Roughly a mile away in each of the four cardinal directions, similarly crafted arrows point away from the circle, whose origin, purpose, and age remain a puzzle."

—From "Journey to the Heart of the Sahara," March 1999, National Geographic magazine