Instructions
Enter Builder Mode via your interface to begin editing the world rather than interacting in it with your character.
Select a thin brick from your inventory, which is unlimited in quantity, and drop it below the "ground" plane that divides "above ground" from "below ground."
Click and hold the blue dots that correspond to each side of the block. Drag them outward to expand the block into a wall. Pull three more thin blocks underground and match their edges up so that they make a square or rectangle.
Drag a flat piece from your inventory to the underground area and stretch its dimensions to completely cover the bottom of the square or rectangle.
Click the "Delete" tool and remove some of the blocks above the tunnel so that players can fit into it. Alternatively, remove blocks at either end of the structure so that the tunnel functions as a quick route between the two ends.