The Hranice Abyss, the deepest known flooded cave, was dissolved by acid rising from below in which country?

Czechia, in the east of the country - and the depth is still not established. A remotely operated vehicle reached 404 metres in 2016, a geophysical survey in 2020 suggested the flooded shaft may run past 1,000 metres, and no one has found the bottom.

It was not made the way caves usually are. Ordinary limestone caves are dissolved by rainwater carrying carbon dioxide picked up from soil, working downward. Hranice is hypogenic: warm, carbonic-acid-rich water rises from deep below and eats the rock upward, which is why the shaft is a near-vertical pipe rather than a branching system, and why the water at the bottom is warm and fizzing with carbon dioxide.

That makes it hostile in an unusual way. The gas concentration, the depth and the total darkness put it beyond conventional diving; the record attempts have used mixed gas and staged decompression running to many hours, and one diver died there in 1981. Almost everything now known about it comes from robots and from sonar, and the current answer to how deep it goes is that the instruments run out before the cave does.