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Prof Stevens described tracking small insects with radar as an "engineer's extreme sport".

"For years and years I was pushing my body beyond its limits physically and mentally. Bodybuilding is an extreme sport and I wasn't healthy," she told BBC Scotland News.

The extreme sport - which involves jumping with a parachute from high landmarks, rather than an aircraft - has become increasingly popular among foreigners living in south east Asia.

Pulling up with a fit communicates to other shoppers that you have your uniform on, and see this as an extreme sport.

But this film, on Netflix, invites viewers to submerge alongside thrill-seekers unconfined by a vessel: Its subject is the extreme sport of free diving, in which competitors plunge into the depths for minutes at a time without scuba gear.