Croatia’s deepest cave system is home to a tiny, translucent resident. The newly named Zospeum tholussum belongs to a group of terrestrial snails found in wet subterranean habitats. Alexander Weigand of Goethe University Frankfurt in Germany retrieved a living specimen from one chamber and a handful of empty shells from others more than 800 meters deep inside the Lukina Jama–Trojama cave system.
At that depth, air temperature drops to a chilly 3.3° Celsius. The delicate shells averaged just 1.55 millimeters tall, or about the thickness of a penny. Unoccupied shells are nearly clear but turn milky white as they age, Weigand reported in August in Subterranean Biology.