Starnawski, 48, dived more than 650 feet down the cavern to a narrow slot before sending a remotely operated robot to go down, the AP reported.
The ROV was able to go down another 650 or so feet, or the length of its cord, but still did not hit the bottom, Starnawski told the AP.
Still, the discovery officially made Hranice Abyss the world's deepest known underwater cavity, Starnawski said. He added that the underwater cave beat the previous record-holder, a flooded sinkhole in Italy called Pozzo del Merro, by at least 39 feet.
Starnawski told the AP that diving in the cave was a challenge because of its muddy areas and the water's mineral composition, which can damage equipment and harm exposed skin.
"But that is the only price to be paid for this discovery, and it was worth paying," he said.
A cross-section map Starnawski made of the cave ends with question marks in an unexplored area where he believes it goes deeper.
National Geographic, which first reported the discovery, covered some of the expedition's cost, the AP reported.