1924
The US Navy begins examining helium's potential usage and by the mid-1920s lab animals were exposed to experimental chamber dives using heliox. Soon, human subjects breathing heliox 20/80 (20% oxygen, 80% helium) had been successfully decompressed from deep dives.[27]
1937
Several test dives are conducted with helium mixtures, including salvage diver Max "Gene" Nohl's dive to 127 meters.[28][29]
1939
US Navy used heliox in USS Squalus salvage operation.[25]
1965
Nic Flemming's work to study sand ribbons in the English Channel became the first studies comparing diver performance while breathing air and heliox in the open-water.[30]
1963
First saturation dives using trimix as part of Project Genesis.[31]
https://en.wikipedia.org/wiki/Trimix_(breathing_gas)#History_as_a_diving_gas