Lukina jama–Trojama, Cave system in Hajdučki kukovi, Croatia.

Lukina jama-Trojama is a cave system in the Hajdučki kukovi mountains that extends through approximately 3700 meters of limestone passages and descends more than 1400 meters below the entrance. The network features multiple large chambers and vertical shafts that create a complex underground landscape.

Slovak speleologists discovered Lukina jama in 1991; exploration continued from 1993 to 1995 under the Croatian Mountaineering Association's speleology commission, and new work began in 2010. The cave is named after Ozren Lukić – Luka, a speleologist killed as a Croatian soldier on Velebit during the Croatian War of Independence. A 2013 survey measured it at 1431 m deep, then the 9th deepest cave in the world.

The system is named after Ozren Lukić – Luka, a speleologist killed as a Croatian soldier in the Homeland War on Velebit. Exploration continued in 1993–1995 under the Speleology Commission of the Croatian Mountaineering Association, and work begun in 2010 is still ongoing. The 1992 descent by Slovak cavers reached only a small lake at −195 m.

Access requires official permits and specialized equipment since the cave lies within Northern Velebit National Park. Visitors should be prepared for technically demanding conditions and arrange professional guidance.

The pools inside contain a large colony of leeches, including a species that researchers identified as entirely new to science. This discovery makes the system an important location for studying specialized underground life forms.