Hi Breeze
The first one is the Stwlan dam above Blaenau Ffestiniog. Owned by First Hydro. To the top right of the photo the tips of Moelwyn slate mine are visible. I think this one is all grilled and gated. A walker died in there a good number of years ago.
The mountain on the left is Moelwyn Bach, the mountain on the right is Moelwyn Mawr. You can see a packhorse track on the bank of Moelwyn Bach in that photo. Beyond the coll between the two mountains is a tramway leading to the Pant Mawr and Fron Boeth slate mines, and the Fron Boeth tunnel. All good stuff :)
The second photo I have no idea but I guess it's some sort of culvert / access for the supply to the powerstation. If you're into drains stick to storm drains unless you want to find yourself generating the power to boil somebody's kettle in Blaenau :lol:
The last two are a Blaenau mine, as J says, on private land.
There are plenty of accessible slate mines within that area; Cwmorthin, Rhosydd, Croesor (small explore unless using SRT / boats), Wrysgan, Conglog, Fron Boeth, Pant Mawr etc. etc.
my orders are to sit here and watch the world go by