Tutorials/Secret door

Secret Painting Door
Start by making a 1x2 gap where you want your doorway to be. Be sure that the wall area around the secret doorway is big enough for the size of Painting you want. For example, for a 2x3 painting, you will need 2 blocks on both sides of the secret doorway, because the doorway counts as 2 blocks. The painting will not stay on the wall unless you follow the next directions carefully: you can either put a sign on the inside of the doorway (not the other side) or you can put up a door from the outside, but make sure the door is closed when putting up the painting. To put up the painting in the right spot, look at the lower left-hand corner block of where you want the painting and left-click on that block (hint: you may need to replace the painting a few times to get the right size). If you choose to put a Door in, DO NOT punch the door open! It will take down the painting. It is recommended to put a Lever on the other side of the painting wall to open or close and lock your door. A way to make the doorway easy to enter is by making a 2x2 hole and putting four signs on the side, then use the sides of the signs to put up a 2x2 painting, allowing a bigger hole to walk through.

Piston Hidden Door
For a door that is completely hidden except the lever/Pressure Plate/Button to power it, you can use a Piston door. A very simple version is one that just moves 2 blocks to the side with 2 Sticky Pistons, which are hidden inside a wall and are powered by Redstone. To make some more complicated and fancy ones, you can get the blocks to move first away from the Player, and then sideways, allowing for 2 wide doors and it also allows you to build it so that the door appears at the middle of a wall, instead of it only being possible to make it either in a corner or in a tunnel of the desired width of the door, because now all the pistons are behind it. You have to do some redstone wiring, though, and if you want to make the door 2 wide it will need at least 12 sticky pistons, 4 to move the blocks into the wall, 4 to move them sideways, and 4 to move the four pistons that moved the blocks sideways sideways. You have to make the delay from the input to the first four pistons, the ones that move the blocks into the wall, 2 ticks. Then split the redstone wires that go to the other 8 pistons; one way has a delay of 1 tick, the other one a delay of 4 ticks. Then after the delays you can join the wires again and bring them to the pistons. To make it more hidden, replace the blocks with bookcases.

Underwater Hidden Door
Doors and entrances can actually be hidden underwater. This is accomplished via the use of water blocking objects such as trapdoors, ladders, signs etc. Many players prefer to hide these underwater entrances in man-made fountains, bodies of water or behind/in waterfalls. When creating an entrance in the base of a body of water dig a small hole, 1 block only is hidden the best. Then place a ladder at the entrance of the hole, you should see that this will block out the water, leaving an air pocket. You can now continue this ladder shaft down into your base. Be carefull when placing torches or light sources to not make this hole easily noticeable.