The Christmas tree is a submarine or shipboard status board that uses a dense array of red and green indicator lights to show the state of various hull openings, valves, and watertight doors. Before a submarine dives, the crew checks this board to ensure all hull penetrations are sealed, indicated by a solid board of green lights. A red light means a valve or hatch is still open.
Heard on the job
We've a green board on the Christmas tree, rig for dive.
Also heard as
- status board