The world’s deepest freshwater cave

Hranice Abyss Cave or “Hranická propast” in Czech is the deepest known freshwater cave in the world, according to Live Science. Geologists think it can reach more than a kilometer below the Earth’s surface, more than twice the depth of the second-ranked freshwater cave.

Hranice Abyss challenges the long-held scientific notion that deep caves form from the bottom up, with warm, acidic groundwater rising up and dissolving bedrock. That’s not how the Hranice Abyss formed, according to research published in 2020 in the journal Geophysical Research: Earth Surface. Instead, evidence suggests that water perforated the cave from above.

Scientists first described Hranice Abyss in 2016 after conducting multiple dives inside the cave. The team then deployed a remotely operated vehicle (ROV) to explore many nooks and crannies inaccessible to divers and measured a maximum depth of 473.5 m. This makes Hranice Abyss the deepest freshwater cave in the world, beating the 392 m deep Pozzo del Merro cave in Italy. However, the recorded depth is limited due to the length of the fiber optic communication cable attached to the ROV.

The 2020 study used gravity and seismic imaging to understand the depth of the Hranice Abyss cave. The results showed that the cave was twice as deep as ROV had ever measured, deep enough to accommodate the world’s tallest building, the Burj Khalifa tower (828 m).

The mouth of the Hranice Abyss cave is a steeply inclined chamber with a small lake at the bottom, according to the latest research. The underwater part of the cave has an irregular vertical cylindrical shape with a diameter of 10 – 30 m. The water temperature in the cave ranges from 14.5 to 18.8 degrees Celsius depending on the time of year.

The expanded map also reveals the bottom of the cave is connected to a nearby sinkhole called Carpathian Foredeep. The sinkhole, located 1.2km from the cave entrance, was formed 19 million years ago and is filled with sediment, meaning it cannot be seen above ground today.

Hranice Abyss Cave was formed after a sinkhole, about 14 – 16 million years ago when water at ground level began to seep through soluble rocks such as limestone. The process of creating a hollow cavity gradually deepens over time, eventually creating a channel for water to flow from the ground to the bottom of the sinkhole. But as sediment blocked the sinkhole, water began to accumulate inside the channel, leading to a water-filled cave.