Hranická Propast world's deepest cave
Hranická Propast - the site of the world's deepest underwater cave Jiří Komárek/CC

The world's deepest underwater cave has been discovered in the Czech Republic, reaching 1,325ft (404m) into the bowels of Earth. The Hranická Propast was first explored in 1999, but only now have divers been able to measure just how far down it goes.

Polish diver Krzysztof Starnawski has been visiting the cave for the last two decades. In an exclusive interview with National Geographic – which partially funded the latest dive – he said the limestone formation that developed was unusual, indicating it could extend down to extreme depths.

When divers entered the water, it made their skin itch. Hot water saturated with CO2 bubbled up, wearing away the rock from the base.

Over the last two years, Starnawski and his team returned to the cave on a number of occasions to measure its depth. Last year, they discovered a passage had crumbled away, meaning they could get a remotely operated underwater vehicle (ROV) even further down. At this time, they reached 1,214ft – just shy of the record-holding Pozzo del Merro cave in Italy, which extends down 1,286ft.

Hranická Propast world's deepest cave
The ROV being positioned to find where the cave ends Marcin Jamkowski

However, when they returned this month they were able to guide the ROV even further. During the last dive, it emerged it had probably hit a pile of debris from the collapsed squeeze passage through which it passed. This time they guided it down to the record-breaking depth of 1,325ft.

Speaking to the magazine, Starnawski said: "The dive on September 27 was one in the long series of dives that I did in the last 20 years in Hranická Propast. They all had the same goal: to explore the cave further and deeper. 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."

Hranická Propast
Krzysztof Starnawski (left) and Bartlomiej Grynda navigating the ROV Marcin Jamkowski

"During this push, the most important part of the job was done by the robot. 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."