SAN DIEGO, a city and port of entry of the United States, chief town of San Diego county, California, 15 miles north of the Mexican frontier. It has a land-locked harbour 5½ miles long and next to San Francisco the best on the Pacific coast of the States, is the selected terminus of the Texas and Pacific Railroad, and has recently become a fashionable winter resort owing to the remarkable steadiness of its winter climate (mean annual temperature 62°). San Diego was founded by Roman Catholic missionaries in 1769. In 1880 it had only 2637 inhabitants, but they have since increased to upwards of 5000. In the county is a lake of boiling mud half a mile long by 500 yards wide.