Use the cutlass on the ivy and pick up the animal. Return to the serpent cave and place the animal on the floor panel. Walk through the trap to the next room. Look at the plug in the ceiling. Go back to the lake above the village.
Piers automatically puts on the diving suit.
Keep walking right until you find the plug. Pull the plug out and return to the lava room. The lava has now cooled so that you can cross. Enter the next room. Look at the crystals.