Dinorwic slate quarry was one of the biggest slate quarries at the end of the 18th century. Even today it is still amazing to see the sheer size of the quarry - they basically 'converted' a whole hillside into slate. Here you can see some inclines and winch houses they built to get the slate down the hill side as well as the massive rubble hills. It is an adventure playground for climbers and really, you are not even oficially allowed to go there at all. From the quarry you have a nice view of Snowdon area. They actually used this slate quarry for some scenes in "Clash of the Titans". More information can be found under http://en.wikipedia.org/wiki/Dinorwic_Quarry