between decks

from The Century Dictionary.

• In the space between two decks of a ship; on any deck but the upper one.
• noun The space between two decks of a ship, or the whole space between the upper and the lowest deck.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical The space between any two decks of a ship
• noun nautical Steerage on a passenger ship in which people who cannot afford a cabin travel

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adverb in the space between decks, on a ship