Dimensional Pockets Mod adds a block that can be “entered” so to speak. The Dimensional Pocket. Right click with an empty hand to enter it. Sneak-right click a wall, floor or ceiling inside the pocket to exit it. You can build stuff in it, for example a storage room or a small base. If you pick up the Dimensional Pocket you can place it down elsewhere, the content will remain. You can even place a Dimensional Pocket inside a Dimensional Pocket.
Credits: Jezza, Nox, gellegbs, NPException