Just follow the white fog in the distance. Its super huge down there with 2 artifacts. Watch out for megalenia and megalosaurus. Only way out is a shaft that ends in the swamp under the castle or the water tunnels leading to pirate cove down the hill from castle.
You can get into the bottom cave a different way then swimming I walked down there, there's not much down there a few dino's and sea pearls.
N artifacts which I guess is worth going there for lol.
My brother and i did! It led to this HUGE cavern with a bunch of water. There is caverns and waterways that lead place. Its so cool. We legit were exploring 3-4hours irl.