Extensive system of underground limestone workings mainly around castle hill. Many are now infilled, however Singer Cavern is accessible and forms part of an underground boat trip, as also some other smaller accessible caverns. The most famous must be the "Dark Cavern" or "144 Cavern", here Sir Roderick Murchison gave his lecture on the Silurian System in 1849. However, recently this famous cavern was filled in by vandals, (the official kind).