He pioneered the making of documentaries filmed underwater and led the development of a type of rebreather.

However, Hass had a formative encounter with the American diver Guy Gilpatric while on a Riviera holiday in 1938[2] which included underwater hunting and photography.

After making expeditions to the Caribbean Sea and writing his first professional articles in 1938-39, in 1940 Hass switched from reading law to studying zoology and graduated with a Ph.D. from the University of Berlin in 1943 at the Faculty of Biology.

[4] Although Don Stewart, one of the first scuba operators on the Caribbean island of Bonaire, blames Hass for single-handedly hunting the Atlantic goliath grouper to local extinction in a book by Callum Roberts,[5] the author clearly refutes that claim later in the same paragraph.

Hass completed his first underwater film called Pirsch unter Wasser (Stalking under Water) in 1940.

Hass was excused from serving in the German military during the Second World War because of poor circulation in his feet caused by Raynaud's disease.

Hass had read the book Die Raubfischer in Hellas (The Pirate Fishers in Greece) written in 1939 by Werner Helwig.

In spring and summer of 1943, Hass stayed for several months at the Stazione Zoologica in Naples and Capri to study and collect Bryozoa, aquatic invertebrate animals, for his doctoral thesis in zoology.

It shows marine life including wrasse, jellyfish, sponges, sea anemones and rays.

[8] In 1947 his film Menschen unter Haien had its world premiere in Zürich, and his most popular book with a very similar title was released in 1948.

He also went on his first expedition with his new research ship named 'Xarifa', which was mostly financed through photo safaris in the Red Sea and by the BBC.

[9] After expeditions in East Africa and South Asia, his first TV series was developed in 1959, in 1961 for the first time about creatures outside the water.