Indeed, in the old sailing vessels cargo was stored at the bottom of the ship and therefore it is feasible that the bottom hatches do serve such a purpose.
This gives rise to a rather comical vision ... Why would any of Hornblower's ships have hatches in the bottom?
We all know what "naval" cargo holds are supposed to look like: they have hatches on top
. Perhaps with some sort of cranes adjacent.
But turning everything upside down is certainly a valid way to put a fresh sci-fi touch on familiar things.
I've thought this for years. It's actually taking up quite a bit of space on the saucer.
If we think beyond the concept of TPTB just using a randomly available set, the obvious analogy here would be the gun turrets of WWII. Quite a bit of vertical structure there, even if mainly for connecting a necessarily topside feature (the gun) with a necessarily deep down feature (the magazines). A two-store set would provide some of that feel, while use of partitions would help with the also desired crampedness.
Could we bow to that analogy a bit deeper, perhaps? In STXI, crewmen are shown loading smallish cylinders into revolver-like magazines that are extremely unlikely to be torpedo tubes. In DS9, the phasers of the Defiant
rely on consumable components of about the same shape and size. What if phasers in the TOS era also consumed a physical resource not unlike a gunshell - perhaps not with each shot, but certainly often enough that there would be a need to involve pairs of hands ready for a reload, and an officer with his hand hovering on an abort button in each firing in case there's a jam.
We could then attribute the height of the facility to it being immediately adjacent to a phaser turret and stockpiling the necessary consumables for the weapon. Small objects, to be sure, but lots of them, and in need of being moved vertically to the weapon above or below, considering how the saucer weapons are positioned.
The other way to use the analogy is to remember that the turret interiors in WWII were dictated by the need of the weapon to move in a specific way (that is, rotate, and have the barrel tilt). We have plenty of reason to believe in retractable phaser turrets in TOS; the vertical space could be for a part of the turret that will come swinging down at retraction, while all the "BoT" action naturally takes place when the turret is extended.
As for the bottom markings, while it would be appealing to consider them mere decoration, they also happen to be the markings that have the best evidence for being something else.
- They are shaped reasonably for something that might open or be jettisoned
- They aren't similar to markings that we know are unrelated to hatches (the thin red lines of the saucer and nacelles)
- Some of them are similar to markings very likely to be hatches (the yellow rectangles of the saucer)
- One of them is actually seen opening in TOS-R
- This part of the hull has known uses for hatches
- Other Trek incarnations have starships with hatches at these very locations