The Gran Quivira unit of Salinas Pueblo Missions National Monument is the biggest of the three units at 611 acres. Prior to Spanish contact, Gran Quivira was a vast city with multiple pueblos, and kivas. Mound 7, a 226 room structure from the Pueblo IV period (A.D.1275/1300-1600), is the largest and only fully excavated pueblo at the site. During the excavation, an older Circular Pueblo was discovered under Mound 7. First contact with the Spanish probably happened in 1583 with the arrival of Don Antonio de Espejo who mentions a settlement that sounds very similar to Gran Quivira. The Spanish returned in 1598 with the expedition of Don Juan de Oñate who was the first Spaniard to colonize what would become New Mexico. Oñate visited a pueblo he called Las Humanas which was the southernmost settlements. As part of the mission system, Las Humanas (Gran Quivira) was first placed under the Pecos Mission District. Later, with the arrival of Fray Alonso de Benavides in 1626, Gran Quivira was given more attention and later became a visita (a satellite mission without a resident Father) of Abo in 1629. That same year construction began on the first permanent mission at Gran Quivira. Under the supervision of Fray Francisco Letrado, rooms on the west end of Mound 7 (the Letrado's Convento) were used by the Spanish for housing and probably an early chapel. Letrado was moved to the Zuni Pueblo in 1631 and Gran Quivira came under the control of Fray Francisco de Acevado at the Abo Mission. Construction on Inglesia de San Isidro was completed in 1635. In 1659 Fray Diego de Santander was permanently assigned to Gran Quivira. Soon after, construction on a new larger church, San Buenaventura, began. By 1672 a combination of disease, drought, famine, and Apache raiding led to the abandonment of Gran Quivira.