Getting to the dark caverns - freeeranger/dark-caverns GitHub Wiki

Getting to the dark caverns is quite a bit more involved than just smacking up a rectangle and igniting it. To get to the dimension, you first need to trade with a cartographer and purchase the map to the forgotten tower, which should be the last trade on master level. It normally costs around 36 emeralds but can be cheaper or more expensive depending on how the villager is feeling, as with all trades. After obtaining the map, follow it and you'll find the forgotten tower after traveling for a while. Defeat all the enemies, claim the loot and steal the key. Then, mine down to bedrock and find a piece of cracked bedrock. When you use the key on it, you'll open a gateway to the dark caverns... Jumping in it will take you to the other side, and to get home just jump into it again from the other side.

TL;DR

1. Purchase forgotten tower map from cartographer on master level
2. Find the forgotten tower
3. Steal the key
4. Find cracked bedrock at the bedrock layer
5. Use the key on the cracked bedrock