Looking past the modeling problem I fooled around and added part of the underground. Here's the hole leading to the first underground section.
I'm not too happy with the transition between the overworld and the underground here. Feels too sudden.
And here's a little into the underground.
Another thing I'm not happy with is the warp pipe here. Took an age to make and it looks way too short, but any higher and I wouldn't be able to jump into it. A good thing though is the pipe is fully functional with the famous sound too and leads to what will soon be the water area.