Yes I guess it's probably man made. Former gravel pit or suchlike.
Swimming is improving by tiny degrees, but I still don't feel any faster, and I still don't feel able to hit that zone like I can in running, where so long as I control pace I can continue indefinitely (until limbs give way). I still don't feel like a swimmer.
Sadly, the Superman physique hasn't started appearing yet, either. I get the feeling that I haven't yet got enough basic technique to be able to knock out endless laps, to be able to develop the muscle strength further, to then develop the technique and fitness even more. It's a virtuous circle, but I'm still standing at the bottom of the moving ferris wheel, trying to jump on.
Having said that, I can feel huge improvements in my technique. I feel like I'm getting towards
Mr Smooth. But as I tire, I quickly become Mr Flail.