If the small outcrop of rocks was ever found and dug out, they would discover a hatch that has been melted and sealed into the foundation, with no seams existing and no way of opening it sort of having a large gathering of the most adept of unicorns operating on it for several days non stop. If they managed to breach the hatch and defeat all the traps laden into the entrance, they would discover the most miraculous of things; a settlement of ponies.