This cavern is completely underwater. You will need to make sure you have the proper equipment before entering.
The Cavern Tower Dungeons are a great place to get loot. Just be careful.
This is the Ruins cavern where you start.
This is the main menu after the mod pack launches.
The caverns are extremely dark. You must have a torch in your hands at all times or you will be in pitch blackness. A cool effect is that you can throw a torch down and it will light up the area below you.