I can be posh and throw here names like village of Llwyngwril (if you can pronounce it :D ) but I better will show the Blue Lake.
It is a very deep lagoon in the old Golwern Slate Quarry above Fairbourne in Wales. It is accessed by foot only through a little wet tunnel.
The pool is coloured blue by the amount of dissolved natural materials within the lake.
Well, it's not one of nature wonders, it is simply leftowers of human actions but still it has its own beauty. It is small, very deep and well hidden in the old Quarry.
See the colour of the water? It was a gray, rainy day when the colour was so bright so I can only magine how unrealistic it looks in ful sunshine.