If you think that a third deck on a ship that looks about 1 and 1/2 decks deep is amazing that third deck had a sort of a well that someone fell down into, implying an even lower deck below!!.
In the 90s, Bill Mumy worked with Innovation Comics (and indie firm that did not last too long) to print a "Lost in Space" title that takes place a couple of years after the end of the series. Mumy wrote it. One thing he did was to have the Jupiter program reverse engineered from alien technology , a "UFO" that crashed on Earth. This helped rationalize how Earth could have such advanced tech by the mid 90s. Okay, getting stupidly silly for a moment, maybe in addition to that "fusion core hyperdrive", maybe humans derived a limited form of "trans-dimensional engineering" (read: TARDIS tech) from the wreckage, thus explaining that impossible lower, crew deck and that "are you kidding me?!" engineering deck...with a chasm like "well".