Hranicka Propast much deeper than believed!

The Czech cave Hranicka Propast bearing the title of the deepest flooded cave in the world, turns out to be more than twice as deep as previously thought! In autumn 2016, a Czech-Polish exploration team using the ROV GRAL Marine remotely operated robot determined the depth of Hranicka at more than 400 m. Now it turns out that the actual depth reaches around 1,000 m!

However, this is not the end of the interesting news from the country of our southern neighbours. It turns out that, according to researchers, the cave was formed as a result of groundwater seeping to the surface, and not, as previously thought, from the surface into successive layers of ground. Scientists believe that this discovery may call into question the origin of other deep caves.

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Hranicka Propast is located in an area where strong karst processes occur, making it resemble a Swiss cheese full of holes. All this is because rocks such as limestone are slowly dissolved by water. Most caves are formed from the surface downwards, when rainwater or water from melted snow, with a slightly acidic reaction from the carbon dioxide dissolved in it, seeps underground.

It then bites into the rocks and forms further cracks, which widen over time. However, deep caves can also form in the opposite direction – from the bottom up, when acidic ground water is sufficiently heated in the Earth’s mantle. The researchers are convinced that Hranicka Propast falls into the latter category, as its waters contain carbon and helium isotopes from the Earth’s interior.

Hranicka Propast has been the deepest flooded cave in the world for several years. In 2016, a Czech-Polish exploration team using a remotely operated ROV vehicle from the Polish company GRAL Marine, established that its depth is 404 m (473.5 m counting with the dry part), which allowed it to take the title of “deepest” away from the Italian Pozzo del Merro – 392 m. Here, however, it is important to note an essential detail, that the cable of the ROV unit prevented further exploration and the true size of the cave and its depth remained a mystery.

Now using a combination of geophysical techniques, scientists have revealed a whole new picture of Hranicka, which is twice as big as we thought. This was after collecting data from a ground-based network of electrodes to measure how easily the limestone conducted electricity – which can indicate areas of rock or fissures. They then used sensors to look for small changes in gravity that might indicate a cave, and finally recorded the reflections of seismic waves created by setting off small explosive charges, a way of creating a special underground map.

The resulting image revealed a system of deep, ditch-like caves, some of which are filled with sediment and have been carved into the limestone, said Radek Klanica, a geophysicist at the Czech Academy of Sciences who led the study

Surprisingly, these sediment-covered trenches extend to a depth of about 1 km below the surface, felt much deeper than previous estimates. The team shared the results of their study in their publication placed in the Journal of Geophysical Research: Earth Surface.

^{Photo: Irena Stangierska}