Deepest subterranean body of water

The world's deepest freshwater cave to be directly measured is the Hranice Abyss situated in the Přerov District, Olomouc Region, in eastern Czech Republic (Czechia) with its water depth most recently calculated by the Czech Speleological Society to be 450 metres (1,476 feet) in August 2022. However, the bottom of the sinkhole has yet to be reached so it's believed to be deeper still.

The Hranice Abyss was first dived by Polish explorer Krzysztof Starnawski in 2015; he reached a depth of 265 m (1,526 ft) but for his efforts required eight hours in a decompression chamber in order to resurface. As a result, it was deemed more sensible to use a remotely operated vehicle (ROV) for future exploration of the cavern, which was done in September 2016. This survey extended the depth of the water to 404 m (1,325 ft), though the total cave depth from the surface was 473.5 m (1,553 ft). The fibre-optic cable attaching the ROV to the surface prevented it from travelling any farther

During the most recent dive, a smaller robot fitted with six cameras and sonar technology was able collect more detailed data pertaining to the cave's shape, enabling a 3D map of its interior to be created.

In 2020, a study of the Hranice Abyss indicated that this cave system may actually extend to around 1 km (0.6 mi) based on data gathered using an array of cutting-edge geophysical techniques including seismic readings from mini explosive charges and a surface-based electrode array that measured conductivity of the limestone below. The results were published in the Journal of Geophysical Research: Earth Surface on 17 August 2020.

The world's deepest known cave is the Veryovkina Cave, located in the Gagra mountain range in north-west Georgia. On the latest expedition into the cave by a group of Russian speleologists in March 2018, its depth was confirmed to be 2,212 m (7,257 ft).