As Monsteri says, the lowest level is water.
However, it is water in 'state only'.
By this I mean that the bottom level does not automatically become a nice blue colour, you have to colour it yourself.

The lowest level is always water.

Okay, I just have a simple question about map building that I can't solve by searching the interwebs. In the map editor, how do you make the ground water? if you just color it blue does it naturally turn into water? Any response is appreciated! :Þ