The mystery of San Francisco is this: the things people love about the City by the Bay — beautiful vistas, social freedom and a sense that anything is possible — remain the same, year in and year out, despite the fact that the city itself goes through cataclysmic changes on a regular basis.

The city that you think you know — that of the Golden Gate Bridge, clanging cable cars and the crookedest street in the world — resists definition with the same amiable doggedness as a Beat poem, an extended Grateful Dead jam or a novel by Dave Eggers. This week, the face of San Francisco might be the Mission, at once the city’s oldest and newest neighborhood, awash in bodegas and bohemia, where, as one resident put it, “I never bothered to get cable, because there were too many interesting things going on outside my window.”

Or it might be the new headquarters of California’s $3 billion stem-cell research program, which the city won in May despite strong competition from Sacramento and San Diego. San Franciscans are excited about the project, which they hope will turn the Bay Area into the Silicon Valley of biotechnology research. But they’re skeptical, too: from the Gold Rush to the dot-com boom, this city has seen its fortunes rise and fall with the times.

Maybe the best way to understand San Francisco is to consider one of its greatest gifts: not the mai tai, the United Nations or even Chinese-food delivery, but “California cuisine,” that unique combination of fresh ingredients and unusual flavors. Step into the place where it all began — Chez Panisse, in nearby Berkeley — and try a salad of blood oranges, black olives and spring onions. There’s no reason to believe these three flavors would complement one another, yet afterward you’ll be thanking the culinary genius who created the combination. That, in a nutshell, describes San Francisco: a place where disparate people, ideas and experiences come together in a pleasing mélange.

That doesn’t mean San Francisco’s blend of cultures came about easily. The arrival of Spanish soldiers and missionaries in 1769 was a disaster for the Bay Area’s native Ohlone people. The little trading town of Yerba Buena became Mexican, then American, following a war between the two countries in 1846. Now called San Francisco, Yerba Buena thrived, thanks in part to the efforts of its Chinese immigrants. Californians at the time had an unusual way of “thanking” those immigrants: they lobbied Congress to pass the Chinese Exclusion Act, which barred anyone from Asia from becoming a naturalized citizen; those who tried were imprisoned on Angel Island.

Another island — Alcatraz, home of the former federal prison of the same name — became a symbol of Native American protest against hundreds of years of mistreatment when members of many tribes occupied it from 1969 to 1971. And San Francisco’s gay community, today the world’s strongest and most influential, was still struggling as recently as 1978, when former city supervisor Dan White shot and killed Harvey Milk, a gay-rights activist who was then supervisor, and Mayor George Moscone in their offices at City Hall.

So how did the Bay Area gain its reputation as the United States’ most diverse, progressive and welcoming city? As a San Francisco real estate broker might say, it’s all about location. On the border between East and West, the city can’t help but draw on the character and traditions of each: it has the mind of a Pacific Rim technopolis and the heart of a Gold Rush boomtown. And if San Franciscans seem to be more flexible and better at accepting — or promoting — changes than most, it’s because they’ve had to be. The discovery of gold in 1848 turned a sleepy trading post of 800 into a metropolis of 25,000 in one year. A 1906 earthquake killed 3,000, left 100,000 homeless and leveled most of the city. And the dot-com boom of the 1990s revitalized the city and sent real estate prices sky-high — until the bubble burst, leaving business leader s scrambling to recover.

There’s a reason the city’s flag shows a phoenix rising from the ashes. San Francisco is constantly rebuilding and re-creating itself, and seems to attract people who want to do the same. “Here, people can reinvent themselves,” said Tim Zahner, public relations manager for the San Francisco Convention and Visitors Bureau. “They don’t have to wear their pasts around their neck.”

And they come from all over. According to Zahner, San Francisco is “the most diverse city in the nation,” with no real majority or minorities. Its population of 30.6 percent Asian, 43.6 percent Caucasian, 14.1 percent Latino and 7.6 percent African-American have settled into neighborhoods that stubbornly retain their distinctive character, resisting the chain stores and consumer culture that pervade other American cities. From upscale Marina and ritzy Nob Hill to fashionable Castro and the oldest and largest Chinatown in the country, San Francisco is still, as Zahner says, “a city of neighborhoods.” Their residents are just as unique, and even more difficult to classify: walk into one of the city’s great restaurants and you’re likely to see a Montgomery Street financier, a Berkeley artist and a bike messenger from the Mission. You might hear them complain about the traffic or the cost of housing, but you’ll never hear them complain about the city itself. People who live in San Francisco are here because they want to be.

And why wouldn’t they? San Francisco Bay doesn’t just provide a breathtaking view. It also moderates the climate to a temperate 40 to 70 degrees all year round. There are more museums, art galleries, concerts and events here than anyone could possibly visit in a lifetime, yet the best attractions are free. There are the redwood groves of Muir Woods, just across the Golden Gate Bridge. There are the 116 square miles of beaches, bike trails, picnic grounds and pastures (yes, even the buffalo still roam here) in Golden Gate National Recreation Area. And there’s the pleasure of simply wandering the city itself.

Maybe that’s why 14.3 million annual visitors sustain a $6 billion tourism industry here. Technology also remains strong, thanks to a concentration of industry leaders — Hewlett-Packard, Apple and IBM all make their home in the Bay Area — and a labor force that’s highly educated, deeply motivated and especially creative. “We have excellent universities, and our public schools are relatively affordable,” Zahner said, adding that the University of California, Berkeley, has more Nobel Prize winners on its staff than any other school in the world.

The city’s business community is also remarkably diverse, ranging from Levi Strauss & Co. — a San Francisco fixture since 1853 — to Lucasfilm Ltd., currently constructing a massive facility for George Lucas’ filmmaking/special-effects/video-game empire on the former Presidio military base.

It’s anyone’s guess whether the new California Institute for Regenerative Medicine, headquarters for the state’s stem-cell research program, will be a shot in the arm for the local economy or a controversial — and expensive — boondoggle. But San Franciscans aren’t afraid of risk. If the project doesn’t work out, they know they won’t have to wait long for another technological breakthrough, artistic achievement or marvel of human endeavor to put a new face on San Francisco.

LODGING

THE FAIRMONT SAN FRANCISCO
Built as a tribute to silver baron “Bonanza Jim” Fair by his daughters, the hotel was almost completely destroyed by the 1906 earthquake and the fire that followed. Rebuilt, it became one of San Francisco’s most opulent palaces, and arguably its most historic: the charter for the United Nations was signed in its Garden Room; Tony Bennett first performed “I Left My Heart in San Francisco” in its Venetian Room; and its $10,000-a-night penthouse was the inspiration for the Arthur Hailey novel Hotel (the TV series based on the book was later filmed at the Fairmont). Accommodations are everything you’d expect from the flagship Fairmont property — down comforters, high-speed Internet access, marble baths in many rooms, and up to 1,200 square feet of space in the suites (with 6,000 square feet in the penthouse). Both the goldcolumned lobby and the Laurel Court restaurant were recently restored. In addition, the Fairmont’s location atop Nob Hill, at the intersection of all the city’s cable car lines, makes it an ideal place from which to begin exploring San Francisco. $$$$
THE FAIRMONT SAN FRANCISCO
950 Mason St.
tel 415 772 5000, fax 415 772 5013
http://www.fairmont.com/sanfrancisco

HOTEL VITALE
One of San Francisco’s newest offerings is the Hotel Vitale, located on the recently renovated Embarcadero waterfront, across from the new Ferry Building Marketplace. The spa-like property features 440-threadcount sheets, limestone bathrooms and a vast array of health and beauty products. Panoramic suites offer 180-degree views of the water and the Bay Bridge from the king-size bed, wet bar or deep-soaking tub. In addition to free high-speed and wireless Internet connections, cordless telephones and storage service for frequent visitors, there’s a complimentary car service for travel within a one-mile radius of the Vitale. $$$$
HOTEL VITALE
8 Mission St.
tel 415 278 3700, fax 415 278 3750
http://www.hotelvitale.com

THE ARGENT HOTEL
Located in the heart of the business district (it’s a short walk to the Moscone Convention Center, the Montgomery Street financial centers and the Sony Metreon entertainment complex), the Argent is a roomy, stylish place to relax after you finish a long meeting. If the state-of-the-art fitness center isn’t enough to help you keep in shape, access to the nearby Sports Club/LA is available, as are in-room massages. Jester’s Lounge is hip enough to impress your guest for a business lunch, and casual enough for you to enjoy it. $$$$
THE ARGENT HOTEL
50 Third St.
tel 415 495 6152, fax 415 495 6152
http://www.argenthotel.com

DINING

CHEZ PANISSE
Berkeley’s Chez Panisse has become a gourmet destination thanks to the philosophy of executive chef and owner Alice Waters: fresh, local food, served in unique and unexpectedly delicious combinations. The restaurant is open, in two seatings, by reservation only, and features a set menu that changes every night (mine featured a savory onion tart, baked Quinault River steelhead with leeks and citrus sauce, spit-roasted chicken with garlic and sherry, chanterelle gratin, butternut squash and broccoli rabe, finishing with a royal apricot soufflé). There’s also an upstairs cafe that offers more options; it’s less formal, but no less delicious. Making reservations a month or two in advance is not a bad idea. $$$$
CHEZ PANISSE
1517 Shattuck Ave.
Berkeley
tel 510 548 5525, 510 548 5049 (cafe)
http://www.chezpanisse.com

THE BLUE PLATE
The Mission is a cool and funky yet warm and welcoming place, so it makes sense that one of the neighborhood’s best restaurants — while cool and funky — serves up food that tastes like Mom’s homemade cooking. Try the meatloaf, served with mashed potatoes and wax beans, followed by chocolate baby cake and a glass of 20-year-old tawny port. $$$
THE BLUE PLATE
3218 Mission, near 29th Street
tel 415 282 6777
http://www.blueplatesf.com

SAUL’S RESTAURANT & DELI
You read it here: there is a delicatessen in California that’s as good as any you’ll find in New York. Not only are the steaming-hot Niman Ranch pastrami sandwiches at Saul’s dripping with flavor, but the intimate atmosphere, with tables just close enough for you to catch bits of everyone else’s conversation, manages to capture the chatty atmosphere of a great deli.Try the chilled borscht as an appetizer, and be sure to order an egg cream on the side. $$
SAUL’S RESTAURANT & DELI
1475 Shattuck Ave.
Berkeley
tel 510 848 3354

SIGHTSEEING
There’s only one moving National Historic Landmark in the United States, and that’s the San Francisco cable cars (tel 415 923 6164, http://www.sfmuni.com www.s,fcablecar.com). Rolling since 1873, the trolleys underwent a $60 million overhaul in 1984.They’re fun to ride — especially after you’ve seen the size of the city’s hills — but they’re also crowded. Fisherman’s Wharf (http://www.fishermanswharf.org), best known for kitschy gift shops and kitschier street performers, offers two can’t-miss sights: the many stalls selling the Bay Area’s signature crab cakes, and Pier 39 (the Embarcadero and Beach Street, tel 415 705 5500, http://www.pier39.com),where as many as 800 California sea lions roar, tumble and laze about on the pier’s “K” dock. The wharf is also the embarkation point for ferries to Alcatraz Island (http://www.nps.gov/alcatraz), a federal penitentiary from 1934 to 1963.Tours offered by Blue and Gold Fleet ferries (tel 415 705 5555, http://www.blueandgoldfleet.com) include visits to the cells of mobsters Al Capone and George “Machine Gun” Kelly (tours sell out well in advance, so book yours when you plan your trip). There are lots of ways to see the Golden Gate Bridge, but perhaps the most interesting is to bike your way across its 1.7-mile span. Blazing Saddles (2715 Hyde St., tel 415 202 8888, http://www.bikethebridge.com) offers hourly or daily rentals and suggested routes across the bridge to beautiful Muir Woods or the little fishing village of Sausalito. On your way, you’ll pass through the Presidio (34 Graham St., tel 415 561 5300, fax 415 561 5315, http://www.presidio.gov), San Francisco’s oldest historic site and a former military post for the Spanish, Mexican and U.S. armies. It’s now a 1,500-acre national park filled with biking trails, beaches and beautiful buildings, including Fort Point (tel 415 556 1693, http://www.nps.gov/fopo), which provides some of the best views of the city and the Golden Gate.

INFO TO GO
Two major airports service the Bay Area.The recent $2.4 billion expansion of San Francisco International Airport (SFO) (tel 650 821 8211, http://www.flysfo.com) included the creation of the nation’s largest international terminal, rental car center and AirTrain rail system. Oakland International Airport (OAK) (tel 510 563 3300, http://www.oaklandairport.com) is served by 14 airlines, including many low-cost carriers. Both Oakland and SFO connect to downtown San Francisco through the Bay Area Rapid Transit (BART) system. BART tickets cost about $5; expect a trip to San Francisco to take 30 minutes from SFO and about an hour from Oakland, with another 15–20 minutes for the AirBART shuttle ($2) from station to airport. The San Francisco Municipal Railway (MUNI) (tel 415 923 6164, http://www.sfmuni.com) operates the city’s subway, bus, trolley and cable car systems. Fares start at $1.25; you’ll need your ticket at the beginning and end of a ride. Free service on all MUNI transportation for seven days is available with a San Francisco CityPass (tel 707 256 0490, http://www.citypass.com).