San Diego is a city in the U.S. state of California on the coast of the Pacific Ocean, approximately 110 miles (180 km) southeast of downtown Los Angeles and immediately adjacent to the border with Mexico. With an estimated population of 1,423,851 as of July 1, 2019, San Diego is the eighth most populous city in the United States and second most populous in California. The city is the county seat of San Diego County, the fifth most populous county in the United States, with 3,338,330 estimated residents as of 2019. San Diego is part of the San Diego–Tijuana conurbation, the second-largest transborder agglomeration between the U.S. and a bordering country, home to an estimated 4,922,723 people as of 2012. The city is known for its mild year-round climate, natural deep-water harbor, extensive beaches, long association with the United States Navy and Marine Corps, and recent emergence as a healthcare and biotechnology development center.