The Beachhead — David Anderson

Fifteen-year old Will awakens in a vast underground bunker called the Beachhead, controlled by a supercomputer named REX. Will, like the other twenty-three teenage occupants, cannot remember his past life and immediately vows to escape. But two years later he's still there. Soon there comes a turning point in Will's life: REX opens an enormous steel door to the outside world.
While other Beachheaders enjoy their newfound freedom on a tropical island paradise, Will plans his escape. Gradually he discovers allies: other teenagers in the Beachhead whom he recruits and builds into an effective team. But who can Will trust and who is going to betray him? There are rival conspirators, lackeys of REX, watching Will's every move. How far are they prepared to go? Can Will and his friends make it past the perilous sub-levels of the Beachhead and through dangerous forest and swamps? What secret weapons is the all-seeing, all-hearing REX about to throw at them?