You'd probably look like this too if you lived exclusively in the freezing groundwater and darkness of one of the deepest caves in the world.

During a field survey in the early 90s, two Slovak cavers discovered an entrance into Slovačka jama, a pit inside the 145-kilometre-long Velebit mountain chain that sits along the northeast Adriatic Coast of Croatia. Over the next decade, researchers from the Croatian University Mountaineering Association would descend into the winding, dipping cave systems that stretch several kilometres inside Velebit to discover a series of chambers, pits and channels, ice-cold pools and streams, and a 513 m free-fall vertical drop - the world's deepest hole.

"It looked like this pit is going to the centre of the Earth. It's like discovering the properties of some other planet, something completely different and unexpected," says one of the team, Dalibor Paar, who descended 450 m down the hole to catch a glimpse of the bottom, dangling on a rope to take measurements of his surroundings. "It is not easy to make measurements in big verticals. Each instrument is connected to a rope, otherwise it would fall down for hundreds of metres."

The team, led by speleologist (the study of caves), Darko Baksic, also found a handful of other cave-dwelling species, including tiny snails, terrestrial isopods, millipedes, springtails and beetles and one super-rare cave harvestman, plus a new species of strange subterranean leech, Croatobranchus mestrovi, at 1,320 m below the surface. "We are talking about animals that are normally living more than 1000 m below the surface. That is really special," says Paar.

Three colonies were eventually discovered, all within 3 km from the other, and one larger than any subterranean leech colony ever found. Most of the leeches were found on rocky walls in thin layers of water, their heads directed towards the current. Some were found at the bottom of the pools, the water temperature ranging from a chilly 4 to 6 °C. Twenty leeches were collected and brought up to the surface for observation, and the species was described in a 2001 edition of Zooligica Scripta.

Thought to be endemic to the Velebit cave systems, C. mestrovi is highly specialised and perfectly adapted it its otherworldly environment. They have no visible eyes, and their flesh can be a translucent milky white, yellow, or pale pink. They have unusually wide mouths surrounded by a ring of compact tentacles, each with five tiny papillae radiating from the tips. Aquatic cave leeches have been found before, in Japan, France, northern Italy and the US, but C. mestrovi's mouth, or oral sucker, is entirely unique. While no one's figured out what the tentacles actually do, the team thinks they probably have some kind of sensory function. It’s also not clear what the leeches eat, but it’s pretty unlikely that there’s any blood down there for them live off like their creepy surface cousins.

The flattened bodies of C. mestrovi move like caterpillars, and are around 4.5 cm long 1.2 cm wide - not including the strange, stiff, finger-like projections that run along either side. These projections are also a total mystery, but the researchers assume they help the leeches to breathe, based on their resemblance to similar extensions on the bodies of jawless leeches, that have a known respiratory function.

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