المؤلفون: Gore, Rick, Clark, Robert

المصدر: National Geographic Magazine, 2004, P. 26.

المؤلفون: Matthews, Samuel W., Magis, Robert C., Parks, Winfield

المصدر: National Geographic Magazine, 1974, Vol. 146, No. 2, P. 149.

المصدر: National Geographic Magazine, 1978, Vol. 154, No. 3, P. 446-447.

المصدر: National Geographic Magazine, 1978, Vol. 154, No. 3, P. 448-449.

المصدر: National Geographic Magazine, 1959, Vol. CXV, No. One, P. 166-167.