This design is super simple and is the easiest invisible door that I've ever seen. Unlike designs that use horizontal pistons, the redstone here is completely hidden underground, and the door itself doesn't create an unsightly dip or gap in the wall.
Here's what the mechanism looks like:
Check out this video of it in action!
Now, let's see how to build it!
First you dig a hole. I've expanded the hole so that I could take pictures for this tutorial, but your actual hole only needs to be one block wide, six blocks long, and five (or more) blocks deep.
Next, lay some blocks on the floor, leaving one empty spot at the far left.
That's because you're going to fill that empty spot with two stacked sticky pistons.
Note: Remember to use sticky pistons. If you use regular pistons, it'll expand just fine, but it won't contract.
Now we're going to add a couple more blocks. It should be pretty easy from the picture below to figure out where exactly to put them.
Now it's time to add your actual mechanism that will make it work. The first, and most important part is to add a repeater pointing at your pistons, set at maximum delay. If you don't set it to maximum delay, it won't work.
Now you can add a whole ton of torches. They move the power around, so that everything happens like it should.
Note that all torches attach to wool blocks. None are attached to the dirt in the background.
Now you just need a little bit of redstone...
And that's pretty much all there is to it! All that's left is to add your gravel or sand that will be your actual door, and to attach a power source to the top.
Voilà! You now have a working invisible door! This design is especially great for Survival maps, because it doesn't require many resources, and will easily blend in with rough, natural surroundings.
If you know of an easier or more effective invisible door, share it with us by making your own tutorial!