For version 3.0 onwards you will also need to install Roughly Enough IDs (whose modid is "jeid" and which is an update of Just Enough IDs).
This is the Carboniferous dimension. The portal block needed to get here and the biomes included are listed below. To open a portal click on it with the bone wand, which has been enchanted with the Enchantment of Time Reversal. The Enchantment of Time Reversal can be obtained from a Palaeobotanist villager, or as a rare chance on the crafting table. Note that the biomes in these dimensions are able to be extremely large, up to several thousand blocks across. Travelling 10 blocks in the overworld will move you 100 in a custom dimension though, and the main mod's config has an option to allow you to increase that if you felt the need. There are achievements for finding all biomes (biomes listed on the same line are regarded as equally valid for those achievements):
Carboniferous: Portal block: Calamites Fibre block
- Carboniferous Deep Ocean / Carboniferous Ocean Escarpment
- Burnt-Out Carboniferous Swamp
- Carboniferous Beach
- Carboniferous Cool Cordaites Woodland
- Carboniferous Estuary
- Carboniferous Gravelly Highlands
- Carboniferous Hills
- Carboniferous Marshland
- Carboniferous Polar Desert
- Carboniferous Polar Desert Ice Spikes
- Carboniferous Polar Lakes
- Carboniferous Shallow Ocean
- Carboniferous Steppe
- Carboniferous Swamp
- Creeks (regarded as subversions of their surrounding biome and not counted towards biome-discovery achievements)