How to Get Underneath atlas using DIFs

Fire up the TSE EA1 demo. Go outside of the fortress then lower the fortress until the terrain goes inside of it. Now enter the fortress, you'll notice that if you can't see the portal leading to the outside world that you can't see atlas.

So if you need to get in a dif underneath atlas you can exploit portals.

Now for step two, raise up the fortress some and then tilt it so that those hallways are underground. If you find the right spot you'll never even know that they were underground. Add a trigger to ignore collisions with atlas terrain to things inside of the DIF and you're set.

You'll have to carefully plan and tweak your method of entrance underground but at least you can get underground without having to add stuff to atlas.

ALTERNATIVE:

You could implement a render to texture on entrance but then have the actually dif somewhere far below the ground, teleport the player to that location as seemlessly as possible and do the same thing with looking out of the cave, building, etc. You wouldn't cancel collisions or anything this way. Hard part would be making everything, bullets, missiles, etc teleport (oh yeah and lighting consistency.