You'll begin the chapter with a short cut-scene. Once you've began the level, proceed straight ahead. You'll quickly notice that you're back to your initial location. Time to try a different route - use the door on your right and get through a short corridor.
You should reach a moss-covered corridor. Turn left and run to the end of the corridor, then turn around and return using the same route. The new chamber contains a large statue - get behind it and dive into the encountered basin.
Make your way forward. The swim is quite lengthy, but you can gather air bubbles that will replenish your oxygen supply. You should finally get some respite by reaching a dry cave. Continue the journey by diving in the new waterway.
The swim should finally lead you to dry land. Climb on the nearby rock and proceed forward. You'll reach a dark chamber inhabited by shadows that are blocking the way forward. Climb on the lit tree to reach the nearby ledge and turn the mechanism to subject the shadows to the light. Use the unlocked pathway to continue.
You'll encounter a new pool - time to repeat the previous steps. Swim through the cave and gather air bubbles in order to replenish your health. The cave will lead you to a large chamber - make your way down the platform to encounter your companion - the fox, who will lead you further into the ruins.
Place the sphere next to a crescent symbol
Get past the branches by grasping the ledge on your left and use it to get to the other side. The fox will lead you to a large chamber. Pick up the sphere from the end of the walkway and plant it inside the circle to unlock the way forward. Make haste, as the revealed door will disappear in a short moment.