Sri Lanka

James Nestor's book, "Deep: Freediving, Renegade Science and What the Ocean Tells Us About Ourselves" begins at the surface and then plunges ever deeper into the unknown – until we are at 35,797 feet below sea level: the lowest point on earth. "Freedivers" come to the ocean to redefine the limits of the human body, swimming up to 400 feet below the surface for minutes at a time in a single breath.

Nestor introduces us to freedivers who are drawn to the sea for a variety of reasons: some to break records, some to find peace, and some who are scientists, freediving 'because it's the most intimate way to connect with the ocean.'

Nestor unveils startling facets of human physiology – most notably the extraordinary life-preserving reflexes known as the Master Switch of Life.

And we learn about the old and new life-forms that inhabit our deep oceans – a habitat with the greatest biodiversity on earth, yet most of it remains unknown.

Abridged and produced by Pippa Vaughan.
A Loftus production for BBC Radio 4.