Kenwood Ladies' Pond, Highgate Men's Pond and Hampstead Mixed Pond (collectively known as Hampstead Ponds), are three former drinking water reservoirs that today serve as popular sites for open air swimming.
There are many benefits to swimming in the ponds compared to a traditional swimming pool - there is no chlorine, they are surrounded by lush trees, and you will frequently spot the local herons flying low and alighting on a nearby branch. In fact the popularity of open air swimming has elevated loyalty to the ponds to almost cult-like status, especially since the pandemic led everyone to appreciate the benefits of the outdoors (just do a quick Google search to see all the articles and blogs on the subject).
On hot days the ponds can get very crowded but they are open all year, and increasingly people are swimming in colder months for its purported health benefits.