Jason Grace saw the tunnel as he, Piper McLean, and Leo Valdez were flying in a helicopter from Mount Diablo, following the rescue of Piper's father. Jason felt somehow connected to the tunnel, but before he could think any more of it the helicopter had flown past.
On the highway, Percy Jackson stumbled upon a woman named June, who asked him to carry her across to Camp Jupiter. He agreed, and carried her there, crossing a river along the way. The Gorgons chased them, but they managed to get through the tunnel with the help of Frank Zhang and Hazel Levesque.
During the Battle of San Francisco Bay, the Twelfth Legion Fulminata defends the tunnel to the last legionnaire until Commodus and Caligula arrive and demand surrender. However, Frank calls spolia opima, single battle to the death between leaders, and the two accept, and Apollo joins in. Commodus faces off against the fallen god, and Caligula takes the praetor. As they fight, they head into the tunnel, and Frank triggers his trap, resulting in an explosion that kills both emperors.
- It was named after Thomas E. Caldecott, a mayor of Berkeley Hills.