Oh my goodness. The underground layer! It is what I've always wanted for my factories, and I can't believe a mod such as this has only been made just now. I'm so extremely eager to try it out!!!
But.
It doesn't seem to work. .-.
"Error loading mods
Failed to load mods: Error in assignID: item with name 'assembling-machine-burner' does not exist.

Source: underground-access (recipe).

Mods to be disabled:
- modmashsplinterunderground (1.0.1)"
My game's version: 1.0.0, build 54889, win64, steam.
At first, I suspected the Space Exploration mod to be the culprit here, as it changes quite a lot of recipes, but after disabling all the other mods the problem still persisted, which is a great shame. That being said, they probably wouldn't work together anyway: Space Exploration adds lots and lots of new planets as surfaces, and I don't expect this mod to have had accounted for that as it wasn't stated explicitly. It would've been GREAT if they were made compatible with each other - two underground layers for each planet... I can only salivate at such a prospect. I guess I'm trailing off into the feature suggestion zone from a bug report, so I'll stop now, sorry. :D
Anyway, thank you dearly for your great work! I imagine you've poured a lot of effort in it, and the (only) screenshot looks amazing - if only I could actually try it out. >.<

P.S. Bet you didn't expect a fatal bug report within an hour of publishing the mod? ;3

nope, quick fix you could add assembling machine splinter will fix shortly

I should have marked that mod as requirement but will add alternate

Ok well that will fix it. Unfortunately I had to replace the recipe as the substitution function did not operate as intended or prior had.
super thankful you pointed this out early as I could have easily enforced requiring ModMashSplinterUndergrounds but that would defeat the purpose of splintering. I'll need to come up with a cunning solution that hopefully wont require using final_fixes or updates to change recipes.
Anyway interesting apparently factorio doesn't care about mod load order anymore or I was just lucky last time.
please forgive modmash is hard to break apart due to everything being somehow ties together

Oh and yes accounted for other surfaces have to enable experimental. Not testes properly so cross fingers.. should work

Thank you! Didn't expect such a quick reply, not to mention a fix! It works now, playtesting. :3
So far, so good, but there's a couple of notes I'd like to share:
1) If the player simply stands adjancent to the shaft, it will keep pushing him back and forth between the surfaces. Maybe add a temporary flag preventing such transfers and resetting once the player leaves the proximity of the shaft?
2) Not to look too picky, but entities and items are missing strings, descriptions are messy, confusing and lacking in grammar. I understand such things are often left unfinished at the early stages of a mod, but please don't overlook these entirely. I can't speak for everyone, but for me, bad writing substantially impacts the experience and might make me leave an otherwise perfect mod. If you would like someone to do the writing for you, I have a bit of a free time on my hands and would be happy to help. :3
3) Some rocks (boulders) underground are undiggable - and, judging by the texture, it is one single type of the same boulder entity. A missing tag, perhaps? It doesn't look enough like a supporting stone pillar or something to be left as an intentional choice for the underground design challenge - though, IMHO, it would make a great idea after some adjustments.

I also have some other suggestions, but this reply to a bug is growing big enough as it is, and I'd better post them as separate threads. Thank you again for your great work! It's a lovely implementation of an awesome concept. :3

Yes a bit to digest. I have noted descriptions of flag.
The boulders are intentional. They are actually radars the the biters will attack/dig, the alternate method was to do it via script and this seemed to be a better solution.
Support pillars is an idea but may take up more UPS and time to do so if at all it will be a later extension