Valley Side is north of the bridge facing the home zone.
It is here on the map. If you swim here you will get sucked to the bottom where you walk for a bit. You will encounter an underwater air dome, then continue down the tunnel. This area on map is green like its grassland but it is mostly a lake surrounded by mountains.
Middle Tunnel
Other Side Lake
Really nothing more than a novelty, but it has me exploring the water areas a lot often. Cheers!