This is yet another caves mod (called Caves)
Warning: alpha version, read carefully...

This mod by default moves most resources into a cave system. Rather than generating ores on the surface, they will only
generate in a cave, which can be accessed via cave entrances. Here the player can mine them, and transport
them out (through cave exits) via conveyer belts. You can configure via settings which resources are in the cave and which are on the surface; the default is that crude-oil is only on the surface and stone is present in both the surface and the cave.

Note: using it along other resource generation mods like RSO will not work.

Cave entrances transport 4 things:
1. Items can be transported via belts. On the left side, items are moved from the surface to the cave. On the right side, items are transported from the cave to the surface.
2. Players are teleported in the middle
3. pollution that is found in the cave is spread outside to the surface.
4. cave entrances and exits act similar to accumulators, power is transferred from the surface to the cave. Just connect
them via poles.

Cave entrances can be created out of wood and stone, and are found on the first tab.
When you place an entrance (and when you mouse over one), you see a preview of the mines so you can see where you could place the entrance.
(you get entrances back if you deconstruct it)

Notes:
This is still an alpha version; so before basing a playthrough on it, please try it first if it works for your configuration. Since the mod so far was only tested in singleplayer, desyncs could be possible.
Also i have so far only tested the early and mid game.
I don't recommend adding or removing this mod on the fly, since i think a clean migration is probably not possible.
(ores on the surface are deleted)
The graphics for the entrance, exit and cave walls are pretty bad. If you have some talent, feel free to create a mod to override them (i would also happily integrate them, but keep in mind that this mod is put into the public domain for everyone to do whatever they like with it)
Transportation of liquids is currently not possible, use barrels on belts instead ( i didn't want to make the cave entrance any wider than it currently is)
Modded ores are supported, as far as i can tell anyway, as long as standard autogen is used. I tested vanilla, bob's and angel ores. The mod simply uses the same settings as for the main surface, so anything that is created there should appear in the cave instead, without any need for configurations.
However, keep in mind that the cave walls can drastically reduce how much of the ores you can reach, and they could be awkwardly placed, so i recommend using larger ore settings.
There are no biters in the cave by default. But beware when leaving the cave since you don't really know if the cave exit is right in the middle of a biter base. You can enable biters to spawn in the cave via a setting. I don't know if new modded enemies work as expected.
RSO will probably not work together with this mod!
The chunk generation used is a bit time-consuming, i had to split it over 16 ticks. For me now i don't notice any lag any more, but let me know how it is for you. cave-generation seems to be lagging in multiplayer however. Note that the cave entrances for the starting area will appear once the cave is generated, which might take a minute. Cave generation is tied to surface generation, so when you explore on the surface, more of the cave is generated (and the other way around too). (also note that factorio seems to generate 1 chunk per second when not exploring, which then means more caves to generate, so if you encouter periodic stuttering, this could be it)
The cave is essentially infinitely large, more chunks are generated on demand.

incompatible mods:
any mod that spawns resources itself, e.g. RSO
AAI pathfinding has trouble finding paths in the caves. playing them together is possible but more frustrating then needed.
(no other mods mentioned so far, but who knows)

mods which work with restrictions:
Building Platform (no platforms will spawn in the cave. also caves must be configured to spawn "building-platform" on the surface.)

credits, notes and thanks:

factorio devs
MagmaMcFry for Factorissimo, which illustrated how to get players, items and power across surfaces
Earendel for Water Fix, where i saw how to create proper round corners with transparencies
Emmote for fewerTrees, for the idea of how to get rid of them (in the cave)
this cave generator is an implementation of
bitbased for the graphics and code suggestions
http://www.jimrandomh.org/misc/caves.html
using the parameters of the Cellular Automata generator from
http://www.futuredatalab.com/proceduraldungeon/
Neemys for supporting with fixing compatibility problems

Version history:
0.1.7 added compability with building-platform mod. note: no platforms spawn within the caves (as of now). you should configure the caves mod so that the "building-platform" spawns on the surface, as per default it would not.
0.1.6 feature: it can now be configured which resources are spawned above and below ground via mod settings. (existing maps keep the "cave-only" setting for all resources)
feature: mine exits can be crafted, placed and mined now, so you can also tunnel out elsewhere. a similar cave-view is present to give a preview where you would end up.
change: to place exits or entrances, now 1 block must also be left below the entrance. (to avoid another getting stuck situation when tunelling out). belts are ignored however so replacing an entrance works in most cases.
0.1.5 bug fixes: fix accidential overworld resource deletetion (affected users who didn't start their map with this mod)
fix north-facing entrance possibly locking the player in the cave. now a 1 wide gap must be left while placing.
fixed the player teleportation placing the player off center
changed the entrance graphics to a better looking one provided by bitbased
cave walls are now slowly minable manually (with a tile in hand). Also a special explosive landfill is added that can destroy walls easily, but it requires a research.
0.1.4 made cave entrances craftable, instead of world-generated. They cost wood and stone, which can be found on the normal surface. You get a preview of the mine in order to know where placing makes sense.
Fixed electricity transportation bug: It was not possible to use solar power effectively because accumulators would not transfer power into the mines.
fixed bug where some tiles where not set correctly in the mines
Note: i added a migration from 0.1.3 to 0.1.4 and for me it worked, but i recommend a backup anyway.
0.1.3 added a mod setting to enable biters in caves. Note that this does not affect existing saves.
0.1.2 increase required factorio version to 0.15.26. i am using features from at least 0.15.13, maybe newer ones too. Nothing else changes so if it worked for you so far, there is no point in updating to this version.
0.1.1 first public release!