John Hopkins

Don, meanwhile, has been at this for decades and is a respected, well-known instructor with a wealth of experience.

Raised in Egham, near London, Don's passion started at the age of 10, after a visit to Wookie Hole in southern England, an impressive cave system featuring a number of underwater chambers and tunnels.

Young Don was entranced.

However, diving was relatively niche in Britain at the time.

Information and instruction were difficult to find and equipment expensive.

Nonetheless, Don was determined.

After leaving school, he joined the army, which gave him the opportunity to learn to dive and try other adventure sports, such as gliding and parachuting.

Don's career in the military took him across the globe, from Northern Ireland to the Falklands and the Persian Gulf.

Alongside his military duties, he racked up thousands of dives.

And as he honed his skills, he began passing on the knowledge he acquired to others.

After retiring from the army at the age of 40, Don was able to focus on his true passion.

He moved to South Africa and set up a diver training center in a flooded, multi-level mine named Kamati Springs.

For divers, it's a paradise of flooded passageways, deep shafts, and old mine workings.

Together with his wife André, Don has built a thriving business.

Among his specialties is his pioneering use of rebreathers, machines which recycle the gas breathed by a diver instead of expelling it into the water like more traditional open-circuit systems.

Rebreathers allow divers to remain at greater depths for longer periods of time, without needing to carry numerous large gas cylinders.

However, due to the contraption's complexity, there is greater potential for malfunctions.

Still, for Don, this tech is liberating.

Don's reputation as an instructor quickly spread.

Among those drawn to his dive center was Australian pilot Dave Shaw.