Messed up Height on map

I need the height in my map to be 0 throughout. How do I do this? I've already tinkered with the height map and Smooth tool isn't working.

WTF? you mean a flat map? lower/raise holes/plateaus, and then find a part you didn't change and you the flatten tool

Yes, you see I have tons of bumps in my map, but I want it to be flat.

I tried using the Smooth tool but that doesn't work, because it just makes it smoother, not flat.

Use plateau, not smooth.

I mean if you're not too far in your map, you could always create a map with a flat surface at the 0 level.

which sucks because then you cant make water with out raising it. always start on 4.