Puget Sound Naval Shipyard caisson in action

This time-lapse video shows how a caisson is used to dry dock a ship or submarine.
Caissons are floating "boxes" that are placed at the seaward end of a dry dock to block the interior of the dry dock from sea, and allow them to be emptied. Caissons can be emptied to float them into place, and partially flooded to seat them into place.
(Distro A: This video was approved for release to the public.)