Tucked away along a shady path towards the north-east edge of Hampstead Heath is a sign: Women Only. This is the Kenwood Ladies' Bathing Pond. Officially opened to the public in 1925, it is the only wild swimming spot in the UK that is reserved for women. Created centuries ago, the Heath's chain of ponds are one of the sources of the River Fleet that runs subterraneously through London. Swimming in the Ladies' Pond's green, silty, silky waters, it's hard to avoid the feeling that you are moving through history and outside of time.