Once a busy slate quarry up in the hills of Snowdonia. As more than a century has passed since its closure, decades of rainfall has caused a man-made lake to form that perfectly reflects the blue sky on a sunny day. Featured in the BBC's program 'Secret Britain' in 2016, it's now not quite as secret as it once was, and almost impossible to get a photo with nobody in the frame during peak seasons - but luckily I was patient enough and rewarded soon after with what was probably the warmest wild swim I've ever had in the UK. What I didn't realise, however was that nobody has ever been able to record the depth of the lake - according to a local, scientists have all broken their equipment trying to descend deep enough to get a reading. Given my fear of extremely deep water, I was pretty glad I only found that information out after I got out of the lake..