The server does indeed have z coordinates down far below zero, which you can discover if you fall beneath the surface of the ground (it's all just water down there). They design the landscapes so that no areas of the map are below 0, I suppose to prevent confusion. Since you can swim down to the bottom of the ocean, they place the lowest point in the ocean at 0 and that raises "sea level" to about 100. Space servers 8192m tall in the z axis, so if the planetary servers are the same size, the landscape is sitting on about 7000m of water.
Sea level on Arkadia is about 85.