Falling through the world takes you to an amazing world way below z = 0 m.
There is this myserious tree over on Next Island which has a crack in one of it's branches. If you land on that crack you fall into the under world. A never ending deep sea with the world, as we know it, floating on islands above you. Mobs follow you but can't reach you. If I remember correct I swam to z = -500 m to be able to swim under a bay of water. A water world below the water. And yes, the water surface is at z = 0 m.
I am sure this don't count, and it was over on that place in the corner anyway.
But I don't think there is any surface you can enter below z = 0 m. Or?