So, Minecraft.

[LennyLen]

I've always wanted to try something like this, but I can't wrap my brain on how to build it underwater and A) not drown, and B) removing all the unwanted water source blocks. Teach me your ways.

For the not drowning part, it definitely helps to have an enchanted helmet with Respiration. The one I have has Respiration III on it, which gives me nearly two minutes underwater. I also recommend creating a few air pockets around your build site before you begin. Placing two blocks vertically with ladders on the sides works well, or if there are any cliffs more than two blocks high, you can carve two-high, one-deep niches out of them to serve as air pockets as well.

If you run out of air, do not try to make for the surface. For some reason, the game knocks you back toward the bottom each time you take damage from no air, so you won't get anywhere (unless you DO have a respiration helmet, which decreases the rate at which you take damage after running out of air). Instead, dig down three blocks and then dig out two blocks vertically next to you at the bottom. This will create an air pocket you can restore your air from.

To remove the water from under the dome, I swam up form the bottom and filled the top two layers with dirt, then dug them out. This created a two-high air pocket at the top. After that, I just removed the rest one layer at a time by replacing the top layer of water with dirt and digging it form above. I discovered that when I removed the layer from two to three metres above the bottom that the bottom two layers of water drained as well, so you can probably just fill every third layer with dirt and then remove the dirt layers from the top down.