The "layers" of the dimension

At the top there's a bedrock layer. This is also where your gateway to the overworld will spawn after you've teleported to the dimension, so keep track of it. Underneath that there's a pretty thin layer of solid carfstone. Then there's the main part of the dimension, the huge caverns, which will take up a huge portion of the world height. Underneath the caverns is an extremely deep and large cave system, with lava and bedrock at the bottom.