Sealab

If you're looking for me
You better check under the sea
Cause that is where you'll find me
Underneath the

Sealab, Underneath the water
Sealab, At the bottom of the sea~

Sealab is located somewhere near the North Beach. Hopefully you are an excellent swimmer, or can hold your breath for a really long time. Not much can be said about Sealab due to spoilers.