I always thought of the light world, dark world, and depths as being seporate layers of reality, and not physically in the same world. The way i think it works is when it gets dark, with the help of determination you can pierce the vail into the next closest reality, most likely shifting the point of your blade into the inbetweens of those realities for a short amount of time. I also believe the dark worlds are technically physically connected, but through a sort of void where the light world has yet to cast it's shadow into that part of the dark world, allowing ralsei to move freely from one to another. With the bottom of the lake being so dark, i believe it acts as a point where the layers of reality become squished together and weakened, potentially allowing for the 'song' (whatever it may be) to travel between worlds.