The Chihuahuan Desert is the largest North American desert. Primarily a desert of Mexico, only a small portion extends across the United States border to meet the mountains and prairies of the north. The desert's position between the eastern and western Sierra Madre ranges in Mexico shields it from incoming oceanic precipitation, so rainfall here averages only 10 inches or less annually. The Chihuahuan Desert receives most of its precipitation from summer thunderstorms sweeping off the Gulf of Mexico in July and August. With its huge expanses, the Chihuahuan Desert is an eco-region that has received less exploration and study than other desert areas. Based on current discoveries, however, it is thought to be perhaps the most biologically diverse desert in the world.