A roll film negative. On board the submarine Sealion (1959) towards the end of a long refit at Portsmouth. The photographer is in the generator room below the main flat between 77 and 82 stations, this space being irreverently known as the snake pit. He is looking aft, to port and down from the centreline at 78 station showing the Caird & Rayner Ltd distilling plant type 'D' with the lubricating oil filter unit on the right. A vertical format negative.