Just another simple copy-n-paste-able design by me.
To activate the door, putting a redstone torch on the bottom 2 blocks from either side of the door to open; it will automatically reset itself after a while. That seems a bit of a secret for not using any button or lever.
It doesn't really have any advantages compare to other door designs alike, but it is pretty compact and it works under water if everything around is covered up, since buttons or levers will be destroyed by water. You can make it as an entrance to the Ocean Monument if you wanna live inside of it or whatever.
Please note that i'm leaving pure mechanism blocks in this schematic. Make sure that those pistons with redstone blocks can extend after pasting. If you don't know how to fix this design, please remind that the bottom 2 furnaces are parts to the circuit function so they cannot be replaced.
~ Thanks to Sphax BDcraft textures for the screenshots.
~ Have fun!