Hranice Abyss

The world's deepest known flooded abyss: a 69.5 m dry shaft above a lake whose water depth was measured at 450.6 m in 2022 without reaching the bottom. It lies in a national nature reserve, and diving is done only by an accredited research team.

Hranice Abyss (Hranická propast) lies in the Hůrka u Hranic National Nature Reserve on the right bank of the Bečva river, in the small Hranice karst between Hranice, Teplice nad Bečvou and Černotín. From the rim a dry entrance shaft about 104 m long, 34 m wide and 69.5 m deep ends at a small green lake, which is only the apparent floor: beneath it a flooded chasm continues to more than 450 m water depth.

The abyss was mentioned in writing as early as 1580 and appears on Comenius's 1627 map of Moravia. Diving began in the 1960s, first on air and later with helium mixes and closed-circuit rebreathers. Polish diver Krzysztof Starnawski reached 265 m on 21 August 2015, the deepest point reached by a diver. Remotely operated vehicles (underwater robots) have gone deeper: 404 m in September 2016 and about 450 m in 2022, still without reaching the bottom.

The abyss formed through hydrothermal (hypogenic) karst processes: warm, carbon-dioxide-rich mineral water rising along deep faults dissolved the limestone from below. Its decorations resemble those of the nearby Zbrašov Aragonite Caves across the river, which formed the same way and are a public show cave.

Diving is not a recreational activity here. Research diving is carried out by the cave-diving group ZO 7-02 Hranický kras of the Czech Speleological Society, working under an exemption from the Czech nature protection act together with university scientists. Visitors can view the shaft from a platform reached by marked trails.

Devonian and Early Carboniferous limestones (Macocha and Líšeň formations). The research group describes two processes: ordinary karst dissolution by rainwater, and hydrothermal (hypogenic) dissolution by lukewarm carbonated mineral water rising along deep faults between the Bohemian Massif and the Western Carpathians, carrying CO2 and traces of helium from great depth.

The water is described as lukewarm, carbonated mineral water heavily saturated with carbon dioxide; the group gives 22.5 °C for the carbonated spring water of the area. A 2022 biological survey using traps found no amphipods, and the researchers concluded the deep water is unsuitable for life.

Deepest known flooded abyss in the world and deepest abyss in Czechia; listed by Guinness World Records as the deepest subterranean body of water (450 m, measured August 2022 by the Czech Speleological Society).

One entrance: the open dry shaft, with the lake at its foot. Access to the water is for the research team only; the public viewpoint platform overlooks the shaft.