There's a large rock outcrop that overlooks Trawsfynydd Lake, its called Carreg yr Ogof (Rock of the Cave), on top of the rock you'll find a "Rock Cannon" - about 32 holes drilled into the rock and all joined together with a shallow chase. These holes and chases were filled with gunpowder (early part of last century) and fired similar to a 21 gun salute on special occassions such as weddings etc.