Query: What is unique about San Diego's harbors?
Context: It is part of the San Diego–Tijuana conurbation, the second-largest transborder agglomeration between the US and a bordering country after Detroit–Windsor, with a population of 4,922,723 people. San Diego is the birthplace of California and is known for its mild year-round climate, natural deep-water harbor, extensive beaches, long association with the United States Navy and recent emergence as a healthcare and biotechnology development center.