Hiya!
If you have access to, or can pick up, these are the books I use regularly when my Players PC's find themselves deep deep deep down...
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1e AD&D Dungeoneers Survival Guide": Great info, IMHO, for getting a DM to think in terms of obstacles and other considerations, like air quality for example. (And for 'inspiration', the last part of the book where it goes into an example of designing an area of the "Lands of Deepearth" in particular).
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Veins of the Earth": Absolutely
amazing book! It's written (mechanics wise) for
Lamentations of the Flame Princess, so expect it to be of mature content (if you are easily offended by speech or have 'delicate sensibilities', you may want to pass on this).
Those are the two "commercial" books I use the most. I also use...
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Trifecta of Twelve": Ok, this is kind of a cheat...it's not an actual product.

It's my own "3d12's" table with sub-tables where I use three different coloured d12's; a "hot", "medium" and "cold" to relate to the colour on the table. I have an example of a slightly old version here:
https://dominions-of-alstigar.obsidianportal.com/ Just go to the "Media Files" link on the left to see the tables. I originally made these to help with running a "Dominion Rules RPG" game, as the page I just linked to shows. But the tables are all generic, so I can use them with pretty much any game.
In all my decades of running dungeon-delves, I've learned one key thing about the "underdark"; Monsters are only half the problem...if that. Running out of food, getting lost, running out of light, running out of...well, all the things that make staying alive possible, actually. Once you start running a deep-delve style game/campaign, you'll start to get a feel for describing the alien world and strange effects that happen to surface dwellers (like COMPLETELY miss-judging the size and distance of large caves...something that looks to be only 100' away could be 600' away; it's a really odd feeling if you've ever been to a major cave system in real life).
^_^
Paul L. Ming