What Is the Deepest Freshwater Cave on Earth?

The Deepest Freshwater Cave on Earth

Hranická Propast in the Czech Republic is the deepest known freshwater cave on Earth, with a confirmed depth of at least 1,553 feet (473 meters) — and scientists believe the true bottom lies even deeper.

Where Is Hranická Propast?

Hranická Propast, also known as the Hranice Abyss, is located near the town of Hranice in the Czech Republic. Unlike most deep caves carved by rainwater and surface drainage, this flooded sinkhole is almost entirely submerged. It drops nearly vertically from the surface, making it one of the most extreme geological features on the planet. The cave sits within limestone karst terrain, a rock type known for dissolving over vast timescales to produce dramatic underground formations.

How Deep Is Hranická Propast?

In 2016, a remotely operated underwater vehicle (ROV) was deployed into the cave to conduct a systematic depth survey. The ROV descended to a confirmed depth of 1,553 feet — but the mission ended not because it reached the bottom, but because it ran out of cable. The true floor of the cave has never been reached. To put that confirmed depth in perspective: the Eiffel Tower stands approximately 1,083 feet tall. Drop it into Hranická Propast and you would still have nearly 470 feet of flooded cave beneath its base.

What Makes the Water Inside So Unusual?

The water filling Hranická Propast is not ordinary groundwater fed by rainfall. It rises from deep geothermal sources far below the surface and is heavily saturated with carbon dioxide, giving it a chemical composition that is highly aggressive toward limestone. This is precisely why the cave has dissolved so deeply — the geothermally heated, CO2-rich water acts as a far more powerful solvent than typical cave water. This chemistry is rare and helps explain both the cave’s extraordinary depth and the continued uncertainty about where it ends.

What Was the Previous World Record?

Before the 2016 survey confirmed Hranická Propast’s true depth, the record for the deepest known freshwater cave was held by Pozzo del Merro in Italy, which reaches approximately 1,286 feet (392 meters). Hranická Propast surpassed that benchmark by a significant margin, reshuffling the scientific understanding of how deep flooded karst systems can reach. The gap between the two record-holders is large enough to swallow a 25-story building.

Why Don’t Scientists Know the True Bottom?

The primary limitation is technology. The ROV used in the 2016 survey was tethered by a cable, and that cable simply ran out before the vehicle reached the floor. Operating at such extreme depths in a flooded, narrow environment is extraordinarily challenging. Water pressure increases significantly with depth, communication becomes difficult, and the geometry of the cave can make navigation and retrieval risky. Future surveys with longer tethers or autonomous underwater vehicles may eventually resolve the question — but as of now, the bottom of Hranická Propast remains unknown.

Why Does the Depth of This Cave Matter?

Hranická Propast is not just a record-breaker for its own sake. It represents a window into deep Earth processes — geothermal activity, subsurface hydrology, and the chemical dissolution of rock over geological timescales. Understanding caves like this helps scientists model how water moves through the Earth’s crust, how geothermal energy transfers heat to the surface, and potentially how life might survive in extreme underground environments. It is, in the most literal sense, a hole in the Earth that science has not yet finished exploring.