I’ve found a way to get under the lake and this is somewhat of an easter egg
To get there you would need to follow the arrow on this picture,
Then afterward you climb up the mountain and near the tree, you would find a hole next to it
You then fall down the hole then follow the path and you’ll find yourself under the lake!
I’ll post more easter eggs when I find them, till then Baii :D. (IGN: Somebody7720)