The Port of San Diego was built in 1542, became a Spanish military fortress in 1769, belonged to Mexico in 1834, was occupied by the United States in 1846, was established as a city in 1850, and was built into a US naval base in 1920. After 1980, it became the main support base for the US 3rd Fleet. The eastern part of the naval base is located in the middle of the bay, with a pier line of 16.5 kilometers and a water depth of 6 to 11 meters. The western part of the base is located in the north of Coronado Island, at the entrance of the Port of San Diego, with an amphibious base and a naval air station. The headquarters of the US Navy's 3rd Fleet and many of its troops are stationed here, and more than half of the fleet's ships include