Velebit
There are hundreds of "holes" on Velebit. It has the largest and deepest caves in Croatia. The three-part "Lukina jama" cave is 1392 m deep, making it one of the deepest caves in the world, and the deepest in southeast Europe, while the "Slovačka jama" is 1320 m. What makes it unique is that it is completely vertical, steepest in the world. At the bottom of the pothole is a water course or siphon with branches that are still unexplored. A kind of leech was discovered in the pothole, which has been ascertained to represent a new species, genus and family; it has been named "Erpobdella mestrovi".
North West Highlands Geopark
The area contains a number of limestone caves. The most well known of these is Smoo Cave, a large sea cave near Durness. This cave is unique within the UK in that its outer chamber was formed by the action of seawater, while the inner chambers were formed by fresh water. Its entrance 15 metres high, is the largest entrance of any sea cave in the British Isles. Archaeological excavations have found evidence of human habitation dating back to the Mesolithic period. The cave became well known after Sir Walter Scott visited it in 1814. Another notable cave site is the Inchnadamph Bone Caves. At these caves, remains of Eurasian lynx, brown bear, Arctic fox, reindeer and polar bears have been found; as well as human remains dated between 4,515 and 4,720 years old. Uamh an Claonaite, Scotland's longest cave is partly located below these Bone Caves.