Upon entering Hot Spring Hideout for the first time, the catwalks just inside the entrance conceal a short tunnel below. The tunnel can be accessed by dropping down into the water and following the watercourse into the rock. Reaching the back of the tunnel requires a character to crouch down at one point.
There are two pirate chests at the tunnel's end. Entering the cave and approaching the two treasure chests will complete the challenge.
- One of the chests is actually visible outside the hideout in the chamber leading to Gee's arena, although is not accessible from this point.