In late September, a team of explorers led by legendary Polish diver Krzysztof Starnawski discovered the world’s deepest underwater cave in the eastern Czech Republic. Named Hranická Propast, the abyss measures at least 1,325 feet (404 m) deep. This makes it 39 feet (12 m) deeper than the previous record-holder, Italy’s Pozzo del Merro sinkhole.
Starnawski has been exploring Hranická Propast for nearly two decades, first visiting the cave in 1999. The highly experienced deep-water diver set the record for the deepest dive with a closed-circuit rebreather off the Egyptian coast in 2011. His fascination with Hranická Propast began after his first dive there. He realized that the cave was exceptional for many reasons, including the strange way it had formed.
What makes Hranická Propast unique?
In a 2015 interview with National Geographic, Starnawski confirmed that the cave “is very unique because it’s like a volcano, formed from hot mineral water bubbling from the bottom up.”
The water is rich in carbon dioxide, creating the acidity needed to dissolve the surrounding rock. According to Starnawksi, only a handful of sinkholes form in this way. The rest form when water erosion from above dissolves porous rock.
Like most sinkholes, Hranická Propast is made from limestone. The water inside is approximately 59 F (15 C). The carbon-dioxide content is so high that it damaged the explorers’ equipment and made their exposed skin itch. However, according to Starnawski, the discomfort was “the price [we] paid for this discovery and it was worth paying.”
Hranická Propast’s unique history first alerted Starnawski to the possibility of extreme depths hidden within its narrow passages and chambers. In 2014, he reached what he thought to be the cave’s bottom after diving to 656 feet (200 m). However, closer investigation revealed a concealed opening into a narrow vertical tunnel, which Starnawksi explored using a probe. He reached 1,260 feet (384 m) before the probe’s cord ran out.
The next year, Starnawski continued exploring the abyss on scuba. The passage had widened due to erosion, allowing him to enter the vertical tunnel in person. He descended to 869 feet (264 m) before being forced to continue with another probe. This revealed a slightly shallower depth of 1,214 feet (370 m). Starnawski hypothesized that a shelf created by a build-up of debris within the tunnel caused this shallower reading.
In September, Starnawski returned to Hranická Propast with a team of Czech and Polish explorers, funded in part by National Geographic. This time, he explored using a Remotely Operated Vehicle (ROV), which can go far beyond the limitations experienced by human divers. He dived to 656 feet (200 m) to set a line for the ROV, then watched from the surface as it descended to 1,325 feet (404 m), smashing the current world record en route.
According to Starnawski, the team was “100-percent sure the measurements were accurate,” thanks to a certified depth gauge on the ROV. However, he also hinted that the cave may be deeper still. He commented that the ROV had not hit the bottom when he recalled it. Starnawski will undoubtedly plan further exploration, but in the meantime he says that the discovery made him feel like “a Columbus of the 21st century.”