The "Biserna" ("Dungeon") cave is the longest and most beautiful cave on the Shumen plateau. The first reports about it date back to 1928 by the French geographer Jean-Denis Barbier du Bocage. Serious research began in 1968 by Shumen and Sofia speleologists. As a result of several expeditions, 2,200 m of the cave were explored and mapped. In 2003, the cave club at the "Madarski Konnik" tourist association in Shumen continued the research and discovered new galleries, whereby the length of the cave "grew" to 2,716 m. The "Biserna" cave is located in the "Kyoshkove" area, 1 km west of the town of Shumen and at an altitude of 404 m above sea level. It is inclined, ascending, branched, two-story and watery, with water constantly flowing out of it. Its entrance is about 50 m lower than the ridge of the plateau and is northeast facing. It is developed in two floors (Upper and Lower Gallery). The lower floor is an underground river, the average flow rate of which is 8 l/s. The galleries are located one above the other and have a permanent vertical connection. The configuration of the "Biserna" cave follows a southwest-northeast development direction.