Mark has 20 years of experience under his weight belt. His pioneering shark-dives and observations have proven an invaluable asset to the global scientific community. He can hold his breath under water for a mind-numbing six minutes. This allows sharks to see him as just another streamlined marine creature instead of a Darth-Vader-like enemy with air tanks huffing through a noisy regulator. Mark owes his breath-holding ability to a heart defect known as Sinus bradycardia. In layman's terms, Mark's ticker beats two-thirds slower than the average male of the same age and physique.