Croatia

Vrelo Une was pushed to -205 m in 2007 and later to -248 m by Luigi Casati. The -248 m figure is an explored depth, not proof of geological bottom.

Crveno jezero was first reached at the lake bottom by Frederic Swierczynski in May 2017 at about -245 m during low water. It is a giant flooded karst pit with open sky above, not a long continuous cave penetration.

Cetina / Veliko vrilo became a landmark connection in 2021 when DIK Freatik divers linked the spring with Gospodska Cave, producing a 4,827 m / 156 m combined system.

Majerovo vrelo is listed by HGSS at 1,020 m / -104 m and is simultaneously an exploration cave and important water source.

Lukina jama-Trojama reaches -1,431 m. In 1994 Zoran Stipetić “Patak” and Teo Barišić dived the terminal sump when the cave was already around -1,392 m deep. Later 2010 and 2013 expeditions extended and surveyed the phreatic continuation. Petra Kovač-Konrad and Vedran Jalžić were principal 2013 sump divers.

The terminal sump is named Patkov sifon in memory of Stipetić, who died in 1996 while testing equipment intended for further sump exploration.

Slovačka jama is -1,324 m deep; Branko Jalžić’s 2019 dive was only about 10 m long and 4 m deep — a perfect example of why underwater distance alone says little about expedition difficulty.

Njemica on Biokovo is another near-1,000 m deep cave with terminal-sump exploration; published cave-depth and underwater-depth datums must be dated and kept separate.

• ›Branko Jalžić “Bančo” — major Croatian cave diver and biospeleologist; Lukina and Slovačka exploration.
• ›Petra Kovač-Konrad — Lukina 2013 and 2021 Gospodska–Veliko vrilo connection.
• ›Vedran Jalžić — Lukina 2013 terminal-sump diver.
• ›Zoran Stipetić “Patak” and Teo Barišić — landmark 1994 Lukina sump team.
• ›Tihomir Kovačević “Tihi” — organiser/explorer in Croatian spring projects.
• ›Luigi Casati — deep Vrelo Une exploration to -248 m.
• ›Frederic Swierczynski — first to the bottom of Crveno jezero in 2017 and later Croatian projects.
• ›PlongeeSout — major 2013-15 Modrić/Zečica survey work.
• ›DIK Freatik — 2021 Gospodska–Veliko vrilo connection.
• ›HGSS — national rescue service with dedicated underwater cave-rescue department.

• ›Lukina jama-Trojama — deep subterranean fauna plus water, sediment, isotope, geology and biology sampling.
• ›Crveno jezero — extreme karst geomorphology, hydrology, microbiology and endemic fauna.
• ›Vrulja Zečica and other Adriatic caves — submerged speleothems as Quaternary sea-level records.
• ›Cetina / Gospodska — physical spring-cave connection constraining the groundwater model.
• ›Vrelo Une and Gacka — major Dinaric groundwater sources where exploration and water protection overlap.

• ›HGSS Cave Rescue Commission — current deep springs, vrulje and pit-sump reference figures.
• ›HGSS — CaveSAR EU Civil Protection Mechanism certification and 2026 training activity.
• ›Croatian Speleological Server / Speleolog.hr / HPS — Lukina and national speleodiving history.
• ›Northern Velebit National Park — Lukina and Slovačka exploration histories.
• ›Hrčak / Speleolog — Zrmanjin ZOV 2007 and 2021 Gospodska–Veliko vrilo connection.
• ›FIAS / Luigi Casati — Vrelo Une -248 m exploration.
• ›Subterranea Croatica / PlongeeSout — Modrić and Zečica 2013-15.
• ›Blue Cave Biševo visitor centre and Croatian marine-science sources — protection and biology.