|Hey, it's that thing you are!|
This map is made to represent what goes on under the main decks. On those mile-long monsters that Space Opera is fond of, I would even make every other deck one of these, or at least 2 out of every 5. Underdeck space like this also gives characters a way to move through the bowels of the ship (no matter how high or low they are) and interact in an environment that is not made for safe passage. It's a trade off: Do you use the main halls and risk running into...everybody, or do you take to the lower passages and risk getting squished, burned, mauled by machinery or shredded by a particularly dim cleaning and maintenance bot? These kind of "rock-vs-hard-place" decisions are some of the things that make dungeon crawls so interesting...
2. Horizontal Shaft: These cross-tunnels are what allow the lift cars to traverse across as well as up and down. The shaft is access able for servicing, but is usually kept in a vacuum in order to increase car speed. Because there is no sound in vacuum, a faint vibration is all the warning interloper get before a lift car slams into them.
3. Air duct: These are the main, super-wide atmosphere circulators that keep fresh air moving in the enclosed spaces of a starship. The ducts are reinforced and able to carry the weight of maintenance crews and robots, and are used as the de facto access passages in the Underdeck areas.
4. Algae tanks: Thousands of gallons of genetically modified algae are circulated through a half-dozen holding tanks in this compartment, where they filter the air and convert the crews waste into edible biomass. It's not the most tasty fare in the galaxy, but the enlisted troops eat a lot of "mystery" protein that began in one of these tanks.
5. Trash Compactor: All non-biological waste ends up in one of these giant garbage mashers eventually, after having all recyclables removed by robots and automatic processes, the remainder is compressed into blocks and jettisoned into open space.