After returning to Arthur with the Water Crest, you will be sent to retrieve the crest located in Thunder Cavern. He will tell you to look for someone who knows how to open the dungeon. This will lead you to the Wastelands south of Pasture Fields, and Durian Village. To get to the Village, enter Wastelands Central from Pasture Fields and make a left. In the next area make a right and your there.
In Durian Village there will be an NPC called Amber close to the town gate that can open the gate, but she asks for 1 each of the following items:
- Ant Leg (Wastelands, Distant Wilderness and Water Cavern)
- Butterfly Wing (Plebian Dunes and Wastelands)
- Spider Web* (Shale Pits, which is east of Neem Town)
Once you give her the items she'll give you some healing items and the Thunder Cavern will be unlocked.