The Valles Caldera is a significant geological feature in the state of New Mexico. The caldera is located within the Santa Fe National Forest; however, the federal government recognizes the caldera as the Valles Caldera National Preserve. A massive volcanic eruption, according to some scientists, occurred within the past few million years, creating the caldera. The diameter of the caldera is an impressive fifteen miles, making it one of the largest in the United States. A caldera is the technical name for the mouth of a volcano. One of the more significant sites, within the caldera, is a lava dome. The build up of magma, under the surface, force the ground upwards in what appears to be a hill in the caldera. As the federal government owns much of the western United States, grazing rights are a critical concern for ranchers in the area. In order to alleviate the demand, the government allows grazing within the preserve through a permitting process. People have inhabited the area around Valles Grande for thousands of years. Archaeological evidence of occupation, as well as hunting, dates the first peoples around 5500 B.C., continuing to the present. The first European explorers reached the site in 1540. While Mexico controlled the area, the Mexican government granted a land grant to the Baca family. During the American territorial period (1846-1912), the Baca family traded their holdings in Las Vegas for equal land in New Mexico. This trade gave the family ownership of the Valles Caldera. The ranch would not stay with the Baca family past the early twentieth century. However, the name stayed the same until the United States passed the Valles Caldera Preservation Act of 2000, officially allowing the federal government to purchase the land and create the Valles Caldera National Preserve. The Valles Caldera Trust, a nine-member board created to run the business of the preserve, runs the preserve itself.