In late June 1961, Mr. Keller and a Life magazine writer, Kenneth MacLeish, dove to a record depth of 728 feet in Lake Maggiore, in Switzerland, on an elaborately built platform that let Mr. Keller experiment with his mixture of gases. At a depth of 350 feet, Mr. MacLeish wrote, Mr. Keller switched to a different mixture of gases he had designed for deeper water.
“There is barely enough ‘air’ to breathe, and it is bitter cold,” Mr. MacLeish wrote, “even colder than the ice water in which we now hover. My teeth itch. I try to say OK but cannot manage it. Still, it appears that I can live on what we are getting.”
Mr. Keller’s success at Lake Maggiore brought him financial support from the Navy. Shell Oil, which was interested in how his research could benefit its offshore oil drilling, provided the support ship for the Catalina dive.
Despite the deaths on that dive, Mr. Keller and Dr. Albert Buhlmann, the cardiopulmonary specialist who had helped Mr. Keller design his gas mixture, signed a contract in 1964 with Shell International Petroleum to continue their research.
“Hannes Keller’s prominence in the world of deep diving was relatively brief but definitely bold,” Mr. Hellwarth said in an email. “His thousand-foot dive turned into a Houdini-like spectacle, unfortunately with disastrous consequences.”
Mr. Keller moved on. In the late 1960s, he and a business partner, Hans Hess, developed a deep-sea diving suit and an aerodynamic ski-racing suit. Over the next few decades he started a line of computers, developed software programs and created an online art and photo museum.
In addition to his daughter Ethel, Mr. Keller is survived by another daughter, Leonie Keller; his sons, Fabian and Severin; and two grandchildren. His marriage to Tzuara Keller-Takahashi ended in divorce. His second wife, Esther (Frei) Keller, died in 2017.