San Diego–Tijuana traces its European roots to 1542 when the land was explored by Portuguese explorers on behalf of imperial Spain. In 1601 it was mandated by the Spanish viceroy in Mexico City that safe ports be found, one of which would be San Diego Bay, for returning Spanish trade ships from Manila to Acapulco. During this mission, explorer Sebastián Vizcaíno was also told to map the California coast in great detail, leading to the further exploration of the modern-day site of San Diego–Tijuana.