This building is known as the Portal to Inferno. It can not be built, and appears on the left side of every main yard. You can only interact with it when you have a level 5 or above town hall. Strangely, the cavern always appears to be level 5. Also, this building looks the same regardless that it's in the Inferno or is in the normal world, meaning that after the last Inferno guarding Moloch base is destroyed, the cavern changes into a portal, rather than a cave as it appears on the "map" of the guarding bases. While the base boundaries won't reach the cavern, it still stops decorations from being put there.