An undersea home has certain pros and cons that set it apart from any other bases or homes. Firstly, an underwater home is immune to any explosions, is difficult to access by Mobs, and can provide excellent visual range and protection. However, an undersea fort can be somewhat time-consuming to build, enter, and exit. Follow these directions and you will have an excellent undersea fortress.
The view from inside a basic underwater house.
Step One
To begin, find a fairly deep spring or lake.
Step Two
Using any Block (e.g. Dirt or Glass), construct any size column equal to or larger than a 3x3 column from the sea-floor to the surface. Dig a hole down the middle of this new column until you reach sand. This is your entrance.
Or you could construct the column 3x3 from sea floor to surface then dig a hole down in the middle of the column as said above until you reach the sand floor. Then dig three more blocks down. Add a ladder to the first sand block which is the sea floor. Go back up and destroy the column and then, since there is a glitch, the water will not go into the hole you made.
Step Three
Construct rooms by first creating the walls and ceiling underwater, then filling it with sand to dry.
Or, if using my design, dig one more block down for counter measures and then build your rooms as said above.
Step Four
Repeat step three as many times as desired. Note that if there is soil on the ground, it will be very fertile, so make crops if desired.
Alternate Options
To begin, find an underwater mound of dirt or stone. Next dig or break a 2 block tall hole in the mound and put a wooden door in the hole to stop the water. Next go in and carve your cave out and you're good.
In SMP, especially if it is set to PvP:true, you want to keep your mines and bases as secret as possible. Find a random spot on the bottom of the ocean. Be sure not to mark it. Start digging a 1x1 shaft downwards (this is easier with a shovel). Come up for air when needed. Dig down until you hit stone. Once you hit stone, mine the first two blocks down, then come back up and start digging horizantally. This will not make your water elevator/ladder go directly into your home. Start digging downwards in a step pattern. Once you think you'll have enough room to make your house, you're good to go.
Tips
- Doors can be placed on the bottom of the water and provide an air bubble.
- If building an underwater tunnel to access your base, bear in mind that still water will allow you to pass in both directions at the same rater, whereas flowing water works better for one-way paths.