A swimmer enjoys a tidal pool known as Pullandase, at the bottom of Kenidjack Valley, West Cornwall, UK. /VCG Photo

The stunning and secret coastal pools sit in a valley that was once an important tin mining area. /VCG Photo

Most of the man-made holes were bored or blasted in the last 50 years, but since the demise of the mining industry they have been left vacant. /VCG Photo

Many of them are right on the coastline and naturally fill with water, and their unique shape makes them perfect natural swimming pools. /VCG Photo