Spanish explorer Melchor Díaz was one of the first Europeans to visit the area around Imperial Valley in 1540. The explorer Juan Bautista de Anza also explored the area in 1776. Years later, after the Mexican–American War, the northern half of the valley was annexed by the U.S., while the southern half remained under Mexican rule. Small scale settlement in natural aquifer areas occurred in the early 19th century (the present-day site of Mexicali), but most permanent settlement (Americans in the U.S. side, Mexicans in the other side) was after 1900.