San Francisco occupies a compact, roughly 47-square-mile peninsula at the northern end of the San Francisco Peninsula, with the Pacific Ocean on one side and San Francisco Bay on the other. It is unusual among American cities in being a consolidated city and county, governed under a single charter and functioning as its own county government. The city traces its modern identity to the California Gold Rush of 1848–49, when it exploded from a small settlement into the principal port and financial center of the West Coast. That mercantile and financial foundation evolved over the twentieth century into a diversified economy anchored by banking, tourism, and, most prominently in recent decades, technology. San Francisco is the cultural and residential core of the broader Bay Area and sits at the northern edge of Silicon Valley, home to a dense ecosystem of software, biotech, and venture-backed companies. Institutions such as the University of California, San Francisco bring a substantial healthcare and research workforce to the city. The city is defined physically by its hills, its grid of distinct neighborhoods, and its famously varied microclimates, with fog rolling through the western avenues while eastern districts stay sunnier. Roughly 825,000 people live within the city limits, making it the fourth most populous city in California. San Francisco attracts a cosmopolitan mix of long-time residents, young professionals, families, and newcomers, and its housing is among the most sought-after and constrained in the country. Despite its density and urban energy, the city devotes extensive land to parks and open space, from Golden Gate Park to the Presidio and oceanfront beaches, and it was the first city in the nation where every resident lives within a ten-minute walk of a park.