Old Llanfair Slate Caverns are a small underground slate working just south of Harlech. Slate was mostly quarried from the surface in this part of Wales, but from 1887 miners also worked underground here, leaving chambers and passages that can now be visited.
The caverns are not as large as the famous slate sites farther north, but they add a good industrial-history stop to Harlech, especially in wet weather or with children who want something more hands-on after the castle and beach.