What is

[perriam]

Fire down below

[bobsailer]

It could mean all sorts of things - including heartburn - but perhaps you're referring to the 1997 Steven Seagal eco-film. FInd out more about it here http://www.movie-source.com/f/firedownbelow/

[andy hughes]

"Fire down below!" is always a serious alarm-call on board a ship. It dates back to the old wooden sailing ships, which often took cannon fire below decks, causing a fire, which was always more dangerous than fires on deck because of the combustable material stored - sails, tar, ropes, and of course guns and ammunition. It has passed into modern language as an indiciation that something seriously wrong has just occured, needing action - now!