The Mizia Plain accounts for almost one third of the size of Bulgaria (32 000 m²). To the west it reaches the lower current of the Timok River, to the south - the northern foot of the Balkan Range, to the north it borders with the Danube River and to the east with the Black Sea. The average altitude is between 100 and 250 m as to the south towards the Balkan Range it reaches 40-600 m. To the north it gradually declines and terminates in a steep bank of the Danube River at an altitude of 100-150 m. At the seaside shore the altitude falls to 290 m at the Frangensko Plateau on the south and to 1-2 m at the village of Darankulak on the north. The Danube Plain is lined by the river valleys most of which are canyons with wide plateaus and flat hills. The river banks often reach up to 100-200 m height. They are vertical limestone rocks with broken surface by hidden natural and man-made caves. Orlova Chuka by the village of Pepelina in the Russe District is the longest natural cave with underground corridors with a total length of 13 437 m. The most famous complex of man-made caves which has functioned as monasteries and churches during the Middle Ages is the rocky monastery by the village of Ivanovo in the Russe District. The walls and the vaults of five churches are decorated with precious mural paintings. They caves are included in the UNESCO Cultural Heritage List.