Polish explorer Krzysztof Starnawski’s fascination with the Hranická Propast, or Hranice Abyss, an underwater cave in the Czech Republic, began in 1999. The diver, who once held the world record for the deepest dive (283-meters) with a closed circuit rebreather, says the cave’s limestone unusual formation led him to suspect that it was a lot deeper than his dives had taken him.