Tunnels! A tavern that was swallowed by the earth but they just tunneled down to it and now its a gimmick. Ancient ruins that intersect with the surface. A shortcut between two buildings. An old mine that could serve as a base of operations. A cult's secret underground lair. A gateway into a splinter dimension that effectively serves as a non euchlidian tunnel. A bit of the road that collapsed into a mysterious freshly-dug tunnel, taking a merchant's cargo with it.