[ I'm giving up on the brackets. Just assume Terry continues to speak in English unless stated otherwise.... ]

Well, the area under the lake is divided into separate floors, so you'd need to swim through the lake portion first to get to the second floor. Then it's like a tundra there. Then you need to fight through a bunch of monsters to get to the next one, beat a landslide, fulfill some weird bear's request, and then you'll finally get to where you're supposed to be. It's a lot of work.