Mike Tomlinson wrote:
"This is because the two reservoirs are linked by one of Britain's
biggest post-war industrial projects: the Dinorwig pumped storage
power station, hidden within this mountain. It is effectively a
monster battery: power is stored by pumping water from Llyn Peris to
Marchlyn Mawr at night, then generated by letting it flow back down
at times of peak demand"

http://www.theregister.co.uk/2016/05...tric_mountain/

I visited it about fifteen years ago, very interesting tour for just a few
quid, you are taken inside the mountain on a full sized 56 seater coach!

The tubes that the water comes down are immense when you get close to them