Velebit caves
The "Lukina jama" cave is 1431 m deep (2013), the deepest cave in Croatia, 14th deepest cave in the world and the deepest cave in southeast Europe. At its foot, there are ponds and streams including one of the largest known colonies of subterranean leeches ("Erpobdella mestrovi"), which has been ascertained to represent a new species, genus and also family. Other species discovered in the system, include the air-breathing land snails "Zospeum tholussum". Like Lukina jama, Slovacka jama, Velebita and Meduza are also located at the "Hajdučki i Rožanski kukovi" special reserve, a dedicated part of the Sjeverni Velebit national park.
Hranice Abyss
The Hranice Abyss (), the English name adopted by the local tourist authorities, is the deepest flooded pit cave in the world. It is a karst sinkhole located near the town of Hranice (Přerov District). The greatest confirmed depth (as of 27 September 2016) is 473 m (404 m under the water level), which makes it the deepest known underwater cave in the world. Moreover, the expected depth is 800–1200 m.