These former reservoirs for drinking water have existed since the end of the 17th century. Hampstead Ponds have several advantages over an indoor pool: there’s no chlorine, you’re surrounded by nature and they don’t tend to get too crowded. There are two single-sex ponds and one mixed pond here (as well as Parliament Hill Lido, an open-air unheated swimming pool). The ponds are life-guarded, the only open-air swimming facilities of the kind in the UK. Note that the lido and mixed pond are closed between mid-September and May, whereas the ladies’ and men’s ponds remain open 365 days a year. Only the very brave might want to swim in these open air ponds in the winter however, but their website has frequent water temperature updates so you can decide in advance whether to dip your toe in. The ponds were free for swimmers since their inception in the 1920s, however there has been a small charge since the mid-noughties.