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Hranice Abyss () is the deepest flooded pit cave in the world. It is a karst sinkhole near the town of Hranice, Czech Republic. In 2020 a scientific expedition revealed that the lowest reaches are sediment-filled and it was speculated that the sinkhole was formed by upwelling hot acidic water. The greatest confirmed depth is , of which is underwater.
At a depth of below the surface of the lake, after crossing the siphon called Zubatice (after its jagged shape), it is possible to ascend to the dry caves ( Rotunda Dry, Heaven I-III, Monika). These are continuously monitored, including water and air temperature measurements. In addition, the Dry Rotunda is known as a roosting site for , which enter it through a narrow passage from the Jezírka area. They have to cross about of rock mass. The occurrence of bats is monitored and studied by experts from the Institute of Biology of the Academy of Sciences of the Czech Republic.
Below the Zubatice is narrow neck opening to the deep underwater cave called Lift. The bottom of the Lift in the depth of is covered by sunken trees and branches making every further movement complicated and dangerous. The area is therefore called Mikado (a reference to the popular game). Behind this obstacle is the neck leading to another deep cave whose depth remains still uncertain.
The report of Jiří Pogoda states that he launched a specially designed probe (glider) from Zubatice to a total depth of on 13 April 1980, during a solitary dive, without reaching the bottom. However, his measurements are incomplete and considered unreliable, as they have not been further verified. In terms of current knowledge, Pogoda's measurements are true, but due to the constriction at , it would be highly improbable for his probe to sink lower and, more importantly, to be able to pull it back through a tangle of logs, branches and other obstacles at that level.
Below the surface, the abyss was mapped to a depth of (Pavel Říha, 2005), followed by a dive to a depth of (Krzysztof Starnawski, 2000) and a 21 June 2012 dive to a depth of (Starnawski with a team of six Czech and Polish divers). Robot (R.O.V.) Hyball was at a depth of (1995). The bottom was not reached because further investigations revealed a configuration of topography preventing further progress of the robot.
The measurement of the depth of the abyss was made on 1 October 2012, when Starnawski launched a probe from a depth of to a depth of during a caving action from the organization ZO ČSS 7-02 Hranický kras Olomouc. He then briefly descended to a depth of himself, which is the deepest depth reached by a diver in this location.
On 12 October 2014, Starnawski again measured the new maximum depth of the submerged part of the Hranice Abyss – . On 27 September 2016, the ROV made by GRALmarine reached a depth of without reaching the bottom. This surpassed the depth of Italy's Pozzo del Merro, which until then had been considered the deepest submerged Rift in the world with a maximum measured depth of . In 2022 UNEXMIN Georobotics reached water depth with the UX1-Neo robot and set the new world record. The total confirmed depth in 2022 was – above water, and below. The depth of the abyss was inferred to be , as indicated by electrical resistivity tomography, gravity, deep‐reaching Magnetotellurics, seismic, and Radiochemistry methods. Further, the previously assumed hypogenic origin (like Zacatón and Pozzo del Merro) was considered unlikely.
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