The Tularosa Basin is bordered on its western edge by the Franklin, Organ, San Andres, and Oscura Mountains, with the fertile Rio Grande valley just beyond, but within the basin water is scarce and water that collects in ancient playa lake beds is often too alkaline to drink. Although the ancient climate was much cooler and wetter, today the basin is a nearly flat desert scrubland about 50 miles wide and 200 miles long. The white sand originates at Lake Lucero at the base of the San Andres Mountains, where seasonal rains fill an ancient playa lake bed and, as the water evaporates, fine gypsum sediments dry and are blown downwind, slowly advancing the vast gypsum dunes farther south and east. About 20 miles to the northwest lie volcanic lava flows called the Malpais, a Spanish term meaning bad lands, created by lava that last flowed as recently as 1,500 to 2,000 years ago and spread southward 44 miles across 125 square miles, reaching a thickness of 160 feet. Despite the rough terrain, the lava flows support diverse plant life and animals including mule deer, Barbary sheep, coyotes, foxes, and birds, while some rodents, lizards, and a type of snake there have adapted darker coloring that helps them blend into the lava, unlike the light-colored forms of the same species in White Sands National Monument. Humans have inhabited the Tularosa Basin for 10,000 years or more, leaving petroglyphs, pottery, stone tools, and village ruins such as those at Three Rivers. European-descended settlers arrived in the late 1800's and used the land for mining, farming, and ranching, but many ranching families gave up their holdings in the 1950's and 1960's when vast tracts were withdrawn for military use, and today much of the basin is occupied by the White Sands Missile Range and Holloman Air Force Base.