Prescribing outdoor swimming for depression is going to be trialled as an alternative to medication.
Scientists want to examine the benefits for people with mental illness offered by ecotherapy - therapeutic intervention through nature.
Experts from the University of Portsmouth will be working with Sussex Partnership NHS Foundation Trust to examine how a dip in nature compares to antidepressants.
Exercise is already recommended by the NHS as a means to help with low moods for people who suffer with depression.
Immersion in cold water has been shown to reduce stress levels, and a search is on for volunteers for the new study who will take part in a swimming course.
Swimming lessons will take place at Parliament Hill in London, Lenches Lake in Worcestershire, and Saunton in north Devon.
The results of the swimmers will be compared against a control group using existing treatments for depression.
It comes as scientists and doctors re-examine their understanding of depression following research that suggested some of the mechanics of how the condition worked may be incorrect.
A spokesman for the University of Portsmouth said: "The study, funded by the National Institute for Health and Care Research (NIHR), will provide preliminary support for using outdoor swimming as an alternative to antidepressants or talking therapies."
Co-author Dr Heather Massey, from the University of Portsmouth's Department of Sport, Health and Exercise Science, said: "In this new study we are looking at outdoor swimming as part of social prescribing, which looks to support members of the community who are self-referred or referred by a number of professional organisations to community activities that will support them.
"It's a step up in terms of scientific rigour."
The mental health charity Mind already recommends ecotherapy.
Its website says: "Ecotherapy is a formal type of therapeutic treatment which involves doing outdoor activities in nature."
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