If your running Android and have Minecraft PE 0.9.5 or below, you can create your own custom flatland maps of whatever blocks you want for however many layers you want!
Step 1: Open File Explorer
First, Open your favorite file explorer and navigate to internal memory, or sd card 0.
Step 2: Open Minecraft Files
Second, find the folder titled "games" that may or may not display the Minecraft icon depending on your file explorer.
Step 3: Navigate to the The Options Folder.
Next, navigate to the options.txt file as shown in the photos above.
Step 4: Modify Text.
After that, edit the highlighted line to modify the map (Explained in the next step). Then save the file.
Step 5: How to Modify Map
Finaly, in order to modify the map you must add to, modify, or remove the numbers in-between the "[" and the "]". Each number has a corresponding block. Just experiment around until you find what you want (I'm sorry I don't have a list, but I updated and now it will not work) Make sure to right down the original numbers so that you can set it back to normal. Also always put a comma "," after each number except the last as shown in the photo.
Warning: the exact numbers in the photo were not tested and may not work.
Step 6: Create the Modified Map
Finaly, make sure minecraft is completely closed (not running in background/multitasking menu). Then create a new flat map. If you do not know how just follow the highlighted button in the photos.
Step 7: Enjoy!
Enjoy your modified flatlands map! This could be used to create a massive underground room, a odd looking ground, a fun game by putting air or a liquid and then sand or gravel above(If you fall in the liquid you lose) or virtually anything.
Note: I say 0.9.5 because that is what I last made it work on. my next update was 0.10.5 and it would not work. If you get it to work on anything later, please tell me in the comments and I will gladly correct it.
Note 2: This will only effect new world's not preexisting ones, also if you want it to return to normal, simply edit the file to the way it was originally.
Warning: if Minecraft crashes (closes unexpectedly), one or more of the numbers you used may not be a block, just change the numbers and try again.