Polish divers are putting their lives on the line to prove an underwater cave in the Czech Republic is the deepest in the world—hundreds of feet lower than depths survivable by humans, National Geographic reports. Krzysztof Starnawski, leader of the dive team, has been exploring the Hranická Propast cave since 1999. Last year, after diving nearly 700 feet into the cave, Starnawski was able to lower a cable another 1,260 feet before the cord ran out—only 26 feet shy of the world-record-holding Pozzo del Merro cave in Italy. But a cave-in sometime between then and a dive in July blocked the team on its return and sent members looking for new passages to prove the cave's depth.
Diving to the depths required in exploring Hranická Propast can be deadly—the body can be overcome by breathing too much carbon dioxide or from water pressure squeezing nerve fibers in the brain—but Starnawski says it's worth it. "I am exploring the unknown, pushing into territory where no one knows what the limits are," he tells National Geographic. "It’s all very interesting and exciting for me." Despite the setbacks and risks, Starnawski is confident the team will prove Hranická Propast is the deepest in the world within weeks. "I will be fulfilled for a few minutes," he says. "Then I will need to find a new project." Read the magazine's full interview with Starnawski here. (Now find out how scientists think another cave can solve a climate change mystery.)