Search the caves for hidden treasure is an optional mission in Ratchet & Clank, taking place in the Tobruk Crater on planet Novalis. Completing it provides a reward in holocards.
Walkthrough[]
Behind the ship lies a small canyon with a pool of water, which leads to a larger pool of water. You can swim in the water by holding 
to dive and swim down, and 
to surface and swim up. Dive below and swim down to trigger the exploding crates, and immediately surface to destroy crates below and collect bolts, though do not swim below for too long, or Ratchet will drown when the oxygen gauge fully depletes. Jump to the grassy surface with a bolt crank and turn it to open the door, revealing a tunnel.
Upon jumping up the rocky platforms in the tunnel, you will encounter amoeboids. These are small enemies that attack at melee range, though striking larger amoeboids simply splits them into two smaller forms. They are best defeated with the Combuster or Pyrocitor. After wiping them out, press forward to find a larger pool of water. Dive below and swim through the tunnel, and then rise up on the other end. Here, you can jump up and drop down to find yourself in another cavern with more amoeboids. Once you clear them out, look to your left to find a hidden area that contains a holocard set. Use the Fusion Grenade to blow it open and collect the reward. Doing so completes the mission.
To return to your ship, head to the pool of water below at the end, then dive and swim through the tunnel below. Upon emerging, step on the green button to open a door, and you will have returned to the starting area.