A team of underwater explorers have discovered what’s thought to the deepest cave in the world. Measuring at around 404 metres (1,325 feet) deep, the cave named Hranicka Propast or Hranice Abyss, was found by Polish diver Krzysztof Starnawski who discovered the cave around 20 years ago in the Hurka u Hranic nature reserve in the east side of the Czech Republic.

For the last 20 years Starnawski and his team have been to revisit the site to fully explore and measure the entirety of the mystical limestone sinkhole. Until September, he has only been able to make measurements at a depth of 370 meters but with the help of a remotely-operated underwater robot (ROV), the team were able to get down much further although still only to 404 meters, it is in fact much deeper still.

The new measurement was enough to beat the current deepest underwater sinkhole that stood at 392 meters located in Italy and known as the Pozzo del Merro.

The dive, which was partly funded by National Geographic, is one of many that excites the team every time they go there due to the unique way the cave was formed. Starnawski explained that it’s like a volcano “formed from hot mineral water bubbling from the bottom up, rather than rain coming from the top down like most caves.”

“There are probably only three caves like this in the world. There is nothing typical about this cave, and every dive we make new discoveries.”

Until new technology can be used to discover just how deep this particular cave is, Starnawski will continue to go after new discoveries that are equally mesmerizing.

“This cave is a big part of my life because I have been exploring it for 15 years, and it keeps going deeper. I want to show people that this is the deepest underwater cave. If I succeed, I will be fulfilled for a few minutes…then I will need to find a new project.”