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 is located. It is also where the Mist (both the natural and magical kind) is the strongest.

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. Zoe 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 the Kampe 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 all go to Mt. Diablo in San Francisco to find Piper's father, Tristan McLean, and fight Enceladus. It is also mentioned that the Roman Camp is located there, the other reason why demigods were not ment to live there.

Trivia

 * Despite being a huge danger to demigods, Annabeth stays there during the school semester, but she did mention she was plagued by a lot of monsters during that time.