This walk in the Nantlle valley visits the 19th century Dorothea Quarry. The slate quarry is now flooded and is a popular place for scuba divers. It's also good for walking with a footpath taking you from the village of Nantlle to Talysarn via the quarry. You'll pass the old pump house and the old Cornish Beam Engine installed in 1904 to pump the pits. It's an interesting place with nice countryside views to enjoy as well.
If you'd like to continue your walking in the area then you could stroll down to Nantlle Lake (Llyn Nantlle) from the village. There's a nice footpath along the southern side of the lake with good views across the water to the surrounding hills.