A team of explorers say they have discovered the world's deepest underwater cave, 404 meters (1,325 feet) down, near the eastern Czech town of Hranice.

Polish explorer Krzysztof Starnawski, who led the team, told The Associated Press Friday he felt like a "Columbus of the 21th century" to have made the discovery.

Starnawski, 48, found the cave Tuesday in the flooded limestone Hranicka Propast, or Hranice Abyss, which he has explored since 1998. He scuba dived to a narrow slot at 200 meters' depth and let through a remotely operated underwater robot, or ROV, that went to the depth of 404 meters.

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In 2015, Starnawski himself passed through the slot and went to 265 meters' depth, realizing that was still far from the bottom and that the cavity was widening. After diving that far down, Starnawski spend must spend four or five hours in a decompression chamber. To explore the cave further, he needed a robot.

Speaking on the phone from his home in Krakow, southern Poland, Starnawski said Tuesday's discovery makes Hranice Abyss the world's deepest known underwater cave, beating the previous record-holder, a flooded sinkhole in Italy called Pozzo del Merro, by 12 meters (39 feet.)

The Czech Speleological Society said it thinks the cave is even deeper and will yield additional records.

On Tuesday, when the robot was 404 meters deep, "it was as deep as its rope could go, but the bottom was still nowhere in sight," the society said in a statement.

Diving in the cave is a challenge because of muddy areas and a water temperature of 7 degrees Celsius (44.6 degrees Fahrenheit.) The water's mineral composition also damages equipment and hurts any exposed skin, Starnawski said.

"But that is the only price to be paid for this discovery, and it was worth paying," he said.

On Saturday, he plans to dive to 200 meters again to bring the robot back through the narrow passage. The device was made especially for the expedition and operated by a Polish firm, GRALmarine.

He said National Geographic, which first reported the discovery, covered some 20,000 euros ($22,440) of the expedition's cost.