I’ve just come across this link from the UK Caving website. Here’s a great little video of a chap re-creating the rock cannons found at Betws y Coed, you can see the video here:
Rock cannons were basically a type of firework display created by the miners making use of black powder strategically placed in holes cut into the rock with channels linking them.
There’s some more info on Wikipedia: https://en.wikipedia.org/wiki/Rock_cannon which makes for some interesting reading as does Griff R Jones book, although good luck getting a copy!!
Loads of people sit on these whilst eating there chips in Betws none the wiser!!