This is how to make a secret base that can't be seen if you're not looking for it.
Step 1: Making Rail Road System
First, dig down 3 blocks and place a regular rail. Dig down in a staircase pattern as far as you want, adding powered rails as you go along.
Step 2: Making Base
Now start hallowing out the base. When you have a hole big enough, start decorating!
Step 3: How to Get Out.
Dig a tunnel two blocks high. Add flooring, walls etc. if you want. Place a door in the tunnel. Open the door and shut it so you are half way through the door. Exit to title, and go back on and you should be above ground! I use this glitch a lot and it always works for me. It's also good for going up really high flat walls if you don't mind losing a door or two
Step 4: Getting In
Place rails along the floor of the base until you hit some thing. Go back to the top and put a mine cart at the top. Place a block of dirt over the hole. I put some flowers around it to mark where it is. Stand on the block and look down and keep looking until the ride button appears at the bottom of the screen. When you see it, click it and you will be in the mine cart. Now ride that sucker into your new base! Before to leave make sure to push the cart back up so you can get back in later. Tada! You have an undetectable base that no one can get into!!!!