Krzysztof Starnawski told The Associated Press today he felt like a "Columbus of the 21th century" to have made the discovery.
Starnawski found the cave Tuesday in the Hranice Abyss, which he has explored since 1998.
He scuba dived to a narrow slot at 200 meters' depth and let through a specially made underwater robot that went to the depth of 404 meters.
He said that that makes Hranice Abyss the world's deepest known underwater cave, beating the previous record-holder, Italy's Pozzo del Merro flooded sinkhole, by 12 meters.