Minecraft wiki entry for Obsidian_farming, associated text: In all cases, be sure to maintain a 4×4 area of flat ground with at least 4 blocks of air above the entire area near the location of the destroyed portal, either by filling in the holes left by mining out the base of the portal each time or by leaving the four base blocks unmined. If this is not done, you may suddenly find the newly created portal is in a cavern underground because there is no longer any valid location for a portal to spawn on the surface.