Hass published "Diving to Adventure," his first book of underwater photographs, in 1939 and some credit him with developing one of the first underwater cameras. Hass completed his first
underwater film called
Pirsch unter Wasser (
Stalking under Water) in 1940. It was published by the
Universum Film AG, originally lasting only 16 minutes and was shown in cinemas before the main film, but would eventually be extended by additional filming done in the
Adriatic Sea close to
Dubrovnik. Hass moved from Vienna to
Berlin in 1941, where he founded the tax privileged society
Expedition für biologische Meereskunde (
Expedition for biological oceanography). 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. From the proceeds of his hundreds of lectures, Hass was able to buy the sailing ship
Seeteufel in 1942. However, he was not able to use the ship for his planned expedition because the ship was in the harbour of
Stettin and it was not possible to bring it to the
Mediterranean Sea during the war. Therefore, Hass rented a ship in
Piraeus and sailed for several months in the
Aegean Sea and the
Sea of Crete in 1942. Before the war, this ship had been owned by the
University of Vienna. During this expedition he filmed and took photos underwater. Hass had read the book
Die Raubfischer in Hellas (The Pirate Fishers in Greece) written in 1939 by Werner Helwig. Hass found this group near
Skiathos and was able to film their
dynamite fishing under water. 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. In February 1944, he completed the thesis to become a
Doctor of Science. Until the end of the war Hass lived and worked in the film studios of
Universum Film AG in
Babelsberg near Berlin to cut and finish his film about the expedition in the Aegean Sea. This 84 minute underwater film,
Menschen unter Haien (Men among Sharks), was released in 1947. It shows marine life including wrasse, jellyfish, sponges, sea anemones and rays. The highlights of this film include
dynamite fishing and interaction of divers with sharks. In Babelsberg he met
Hannelore Schroth, a famous German actress. Hans and Hannelore married in 1945. ==Post war activity and fame==