For version 2.01 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 Cambrian dimension. 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.
