I just noticed that there are two Sideways Caves in the vanilla game, the original of which is not attached to the rest of the world.

Are there any mods that make use of that cave? Two zones, lots of water.

Is the unused one already "finished" with loot, enemies, etc.?

Seems to be. Chests placed, so you get the regular loot for those chests, and a bunch of levelled list creatures placed.

COC to SidewaysCave, or SidewaysCave02 to see them.

The ones actually used in the game are SidewaysCaveNEW, SidewaysCaveNew02 and SidewaysCaveNew03.

I thought they might be active in FCOM, but TES4Edit is showing nothing for them.

If nobody is using them, would anyone like to suggest a location near the water and maybe we can get them active for OOO or something.

in the western bank side of Wellspring island, so you can swim to get there?

Perhaps it was a developer's testing area for the cave-like dungeons? If so, some modders could possibly make dirty edits to that cave, so cloning it to a new cell would be advised.

You mean, in the side of Wellspring Island, but not connected to the Grove or Wellspring Cave?

^Sounds like a good idea. I don't think many mods alter the area around wellspring island, but there are ULs which would need patches if it was elsewhere.

Yes, so it's somewhere beneath the island. I reckon Wellspring cave is beneath the seabed east of the island. They could be connected too, I always found Wellspring cave a bit boring as it is.

I've long thought that the water levels around the IC have raised since the Ayleid's days (though the Imperial Island desperately needs to be sunk down...it's too high up right now), so just below the waterline might be an interesting place for a cave. Need to look properly at the cave before anything is done to see if it needs a touch up.

If they were connected, it would have to be in a way that doesn't mess up the quest section of Wellspring Cave.

Something will need to be done to differentiate it from the real Sideways Cave because in the CS it's almost impossible to tell them apart.

I just did a quick run-through. The variety of creatures seemed larger than normal so they were fighting each other a lot, but the layout is pretty good and everything seems functional (except the door to enter/exit). Have to see if someone can put it to good use.

I like that idea. Morrowind has a couple of entrances like that - and KotN has one of course (which seems to strengthen the water level theory).