The Sector Five Security Countdown was a program set up by Franz Hopper when he created Lyoko. The program initiated a 3-minute countdown whenever someone entered Sector Five, though the countdown does not technically begin until the wall of the dome opens. This was a time limit for finding a hidden key somewhere within the Core Zone, that, when activated, stops the countdown. Tripping the key mechanism allows access to the Elevator, which in turn allows access to other areas of Sector Five such as the Celestial Dome, the Skid Hangar, and the single tower seen in Franz Hopper.
If the countdown ran out before the Key was tripped, the Core Zone would revert to its original, unopened structure, potentially trapping or devirtualizing anyone left inside, as shown with Aelita in Exploration and Yumi in Is Anybody Out There?, respectively.
When Aelita and Jeremie recreated Sector Five in William Returns, they removed this procedure so as not to be further hindered in their fight against X.A.N.A.. The key mechanism makes a reappearance in the Sector Five Replika, where instead of serving to stop a countdown, it is used to deactivate the trap set on the Skidbladnir.