Being local (Newbury) I know it well, I also spent 10 years of my life working on the common,when it was an active cruise missile base for the USAF 501st. If you are going to do that sort of thing you need to pick the right pond to cycle into...Some of them were created when HUGE underground fuel tanks were excavated as part of the return to heathland project after the base closed, and as a result are VERY deep indeed, the sort of very deep that would cause you to drown! Pick the wrong 'pond' to cycle into and you'll be disappearing into 15' of water, surface area isn't a safe guide either.