This mod, adds the under! it's really expensive, since you make the portal out of BEACONS!
And also, to light it, you'll need the under portal lighter [​sandwich 2 beacons between a enchantment table]
The ender basically is a really barren end dimension [​basically you'll spawn in the void and you can find small islands]
And the dimension is a good one for building a spaceship / floating island survival world!