On May 27, 1939, Owen P. Churchill made the first operational helium-oxygen or Heliox dive. The rescue and salvage of the submarine Squalus has been one of the greatest achievement in diving. 33 people were evacuated from a depth of 242 ft. The procedures were theorized and tested on animals almost two decades before by Joel Henry Hildebrand with R. R. Sayers and W. P. Yant, but never attempted.
