A group of Polish cave divers have stumbled upon what would be the world’s deepest underwater cave. The cave is called Hranicka Propast, and it was recently examined with an underwater robot in the Czech Republic.

While scientists have always known this mysterious cave to be deep, it was until a team of spelunkers took a closer look that they realized just how astonishingly deep it was. They have measured it at 1,325 feet deep, which would make it the deepest cave yet discovered on Earth. The previous record holder is Pozzo del Merro, a cave in Italy that is 1,286 feet deep.

This isn’t your typical diving scenario, so the team needed a remote operative vehicle to access this cave. Still, scientists have dived their before — many times over the years, in fact. It has often been explored because it was formed from hot mineral water bubbling from the bottom, and not from rain coming down as is the case in most caves. It’s a very unusual geological feature.

Polish diver Krzysztof Starnawski, who led the expedition, told National Geographic that the Sept. 27 dive was one of a series of dives in the last 20 years at the abyss.

“They all had the same goal: to explore the cave further and deeper,” he told NatGeo. “As the expedition leader for the last several years, I’ve prepared the equipment and the route in and out for the ROV’s dive, so the ROV could go beyond the limits of a human diver, and get through the restricted passage and between the fallen logs and trees.

“During this push, the most important part of the job was done by the robot,” he continued. “I scuba dived down to 200 meters just before the ROV’s deployment to put in the new line for the robot to follow. The goal was to give the ROV a good start from there to the deepest part of the cave. I came back to the surface, and then we went down with the robot to a depth of 60 meters (197 feet). From there, the team at the surface navigated it, via fiber-optic cable, down along my new line to 200 meters deep. Then it went down to explore the uncharted territory—to the record-breaking depth of 404 meters.”