text: Explorers in the Czech Republic have just completed an expedition to the underwater cave Hranická Propast - also known as the Hranice Abyss - verifying for the first time since its discovery in 1999 that it really is the deepest underwater cave on Earth.
  The team, led by Polish diver Krzysztof Starnawski, managed to reach a depth of 404 metres (1,325 feet) using a remotely operated vehicle (ROV), which means this limestone cavern is 12 metres (39 feet) deeper than the second deepest underwater cave that we know of - Italy's Pozzo del Merro.
  The most recent expedition, which was sponsored in part by National Geographic, is the result of over two years of diving. Back in 2014, Starnawski thought he had reached the bottom of the cave at a depth of 200 metres (656 feet).
  As Kat Long reports for National Geographic, with further investigation, Starnawski found an extremely narrow 'squeeze passage' that gave way to a gigantic, pitch black, vertical tunnel, which he tried to explore with a probe, but ran out of line at a depth of 384 metres (1,260 feet).
  In 2015, Starnawski conducted another dive in the cave, revealing that the passage he managed to stick a probe through last time had widened, which allowed him to actually swim through.
  While inside this black abyss, he managed to reach a depth of 265 metres (869 feet) where he unleashed another probe, finally managing to reach somthing solid at a total depth of 370 metres (1,214 feet) - possibly landing on top of fallen debris.
  According to Long, during the most recent expedition last week, the team managed to use an ROV to fully explore the cave, verifying that it is, in fact, the deepest known cave in the world, at a depth of 404 metres (1,325 feet).
  "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," Starnawski told National Geographic.
  For this expedition, he dived down to 200 metres (656 feet) to affix a guideline for the ROV to follow. His team deployed the ROV, and he accompanied it down to 60 metres (197 feet).
  "From there, the team at the surface navigated it, via fibre-optic cable, down along my new line to 200 metres deep. Then it went down to explore the uncharted territory - to the record-breaking depth of 404 metres," he told National Geographic.
  "The ROV that reached 404 metres has a depth gauge that was tested and certified by our state commission, so we are 100 percent sure the measurements were accurate."
  While the team is sure that the cave is the deepest known in the world, there are still many things to learn about it. It's thought that the limestone cavern was created by bubbling hot mineral water filled with carbon dioxide that slowly formed a tunnel over time, but further research is needed to fully understand its formation.
  "This cave is very unique because 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," Starnawski explained to Mark M. Synnot at National Geographic last year.
  "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."
  Check out the video below of Starnawski diving into the Hranická Propast last year: