[1] On 18 August 1934, Cosyns together with his student Nérée van der Elst piloted a balloon to an altitude of 52,952 feet.

They were unsuccessful in maintaining satisfactory radio communication with ground, but were able to make observations of the currents in the stratosphere as well as investigate the nature of the cosmic rays.

[2][3][4] A large bronze monument in the shape of a balloon was erected in 1997 on the spot of their landing in Ženavlje to commemorate the event.

In 1952, he was in charge of the speleological expedition to the Gouffre de La Pierre Saint-Martin cave system in the Pyrenees.

The French speleologist Marcel Loubens, died in an accident with an electric hoist during the ascent after spending four days in the cave when the steel cable snapped.