Cave of Devetaki

The cave of Devetaki is situated on the banks of the Osam River, 2 km north of the village Devetaki, about 20 km north-east of Lovech, about 55 km north-west of Pleven and 30 km north-east of Levski
The cave is 2440 meters long, which makes it one of the largest in the country. The biggest cave hall in Bulgaria with a length of 360 meters is set in it. The cave has galleries, lakes and springs. In excavations carried out in it were made open to some of the most interesting remains from Neolithic times in the Bulgarian lands. Interestingly, it is closed in the period from 01 June to 31 July due generative period of bats that live in it. 
There are two main avenues to reach Devetashka cave. One of them passes deflection in the village Devetaki. From there began a dirt road that the entrance of the cave. The other road wean again from the main road Devetaki-Letnitsa in the area between the villages Doyrentsi and Alexandrov. The road is paved to the bridge and then continue walking on the trail. 
The cave was discovered in 1921 and was declared a landmark in 1996. 
Sources:, and 

06.12.2010, Bulgarian sights