After graduation, he moved to Poltava for a job in the regional branch of the All-Union Research and Design Institute for Explosive Geophysical Survey Methods.

[5] Kasyan continued as instructor at the Poltava regional center of education in tourism and local history[6] and from there moved on to a career in industrial rope access.

Cavers are known for their superstition,[18] and Kasyan was never tempted to estimate expedition potential in advance: Openly talking about cave depth (you are going to achieve) is a very bad sign.

[18]Kasyan's most important achievements are connected to exploration of a cave system above the eastern coast of the Black Sea, in the Ortobalagan valley in Abkhazia.

[19][20] He was leading the following expeditions, nearly always organized by the Ukrainian Speleological Association: His right hand in most of these projects was Russian cave diver from Simferopol, Gennadij Samohin; as of July 2019 cavers from 20 countries have participated in the list (sorted in descending order by the number of expeditions – given after the country name if exceeding 1): Ukraine (21), Russia (14), Israel (8), United States (4), Bulgaria (3), Lithuania (3), Poland (2), Spain (2), United Kingdom (2), Belarus, Belgium, Britain, France, Iran, Ireland, Italy, Lebanon, Moldova and Turkey.

[47][44][45][46][3][48][49][27] Kasyan also made a major contribution to exploration of caves in the Aladaglar mountain ridge above the southern coast of Anatolia in Turkey, in the framework of the Call of the Abyss project.