A Polish explorer claims to have discovered the world's deepest underwater cave, 404 meters down, near the eastern Czech town of Hranice. <br/><br/>Krzysztof Starnawski found the cave on 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.<br/><br/>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. Starnawski said he felt like a "Columbus of the 21th century" to have made the discovery.