San Francisco

San Francisco is where Annabeth and her family live. It is also where the Garden of the Hesperides is located, where Atlas holds up the sky, and where the Titans' base (Mount Othrys) is located. It is also where the Mist (both the natural and magical kind) is at its strongest. Camp Jupiter is said to be located around this area, but it's exact location is currently unknown.

The Titan's Curse
San Francisco was first mentioned by Annabeth when she told Percy how her dad was planning to move there for his studies. When Percy asked why she was so reluctant Annabeth told him that every demigod should know that San Francisco is the most dangerous place a half-blood could be. Thalia later explained to Percy that San Francisco was where the Mountain of Despair lies (which is why the mist is so strong), and where a lot of monsters dwell. Zoë Nightshade, Thalia, and Percy had to visit San Francisco and seek help from Annabeth's dad to get them to the mountain.

The Battle of the Labyrinth
When Annabeth, Tyson, Percy and Grover enter the Labyrinth, they emerge on Alcatraz Island on San Francisco. They save Briares, a hundred-handed one, but the prison was also where Kampê is. Percy mentioned that the Mountain of Despair had dark storm clouds swirling around it, saying that there's huge trouble brewing.

The Lost Hero
Jason, Piper, and Leo are transported there by Aphrodite, they all go to Mt. Diablo in San Francisco to find Piper's father, Tristan McLean, and fight Enceladus. It is also mentioned that Camp Jupiter is located there, the other reason why Greek demigods were warned not to go to, live in, or even be near this area.

The Son of Neptune
Having been swapped with Jason and his memory stolen by Hera, Percy meets Lupa at the Wolf House and is told to use his senses to find Camp Jupiter. For three days, Percy has been chased and attacked by the Gorgon sisters, Euryale and Stheno after they met at a Bargain Mart.

Trivia

 * Greek demigods are warned to stay away, due to the large amount of monsters located around the Mount Othrys area.
 * Despite the area being a huge danger to demigods, Annabeth stays there during the school semester, but she did mention she was attacked by a lot of monsters during that time.
 * The San Francisco Bay area is home to Camp Jupiter, the exact location remains unknown. The Roman camp could have been created there to keep watch on the Titans.