From Wiki:
"RMS Columba was the last of three car ferries built in 1964 by Hall, Russell & Company, Aberdeen for the Secretary of State for Scotland.[7] The Secretary of State for Scotland ordered a trio of near-identical car ferries for the Western Isles. They were chartered to David MacBrayne Ltd and were all equipped to serve as floating nuclear shelters, in the event of national emergency.[8] This included vertical sliding watertight doors that could seal off the car deck, immediately aft of the hoist."
[8] Refers to:
https://web.archive.org/web/20171002215 ... ythonplan/This takes a while to load. Must be stored in the depths of a nuclear bunker.