In 1967, at the encouragement of his fellow instructors, Croft set out to discover how deep he could dive while holding his breath.
Over an 18-month period, in competition with Jacques Mayol and Enzo Majorca, he established three depth records: He retired from free-diving thereafter.
He had developed this method as a youngster living in Narraganset Bay, Rhode Island to swim further and stay underwater longer than any of his peers.
One of the navy research teams, Dr Karl Schaefer (US Navy) and Dr Robert Allison of the Scott White clinic in Tempel Texas, had done research on diving mammals that demonstrated that air-breathing animals could go to half a mile and deeper without experiencing thoracic squeeze.
This led to the published study: Schaefer, Karl E., Allison, Robert D., Dougherty, James H., Jr., Carey, Charles R., Walker, Roger, Yost, Frank, & Parker, Donald (1968).