How many times have you visited a military facility as a tourist? Not very often is our guess. Well, Rijeka offers a rare opportunity to do exactly that. And it will lead you underground in the process. The first military tunnel opened exclusively for tourist visits is waiting for you in the very centre of the city.
The decision to build the tunnel arose from the fact that the city sprung up at the crossroads of several historical states, which is why it also became an area of particular military significance. The frequent shifting of borders led to the construction of fortifications. Most of them were built in the years leading up to World War II, during the construction of the so-called Alpine Wall (Vallo Alpino), which was supposed to protect the border between the Kingdom of Italy and the Kingdom of Yugoslavia. The construction of subterranean strongholds, military warehouses, passages and bunkers began in 1931 in order to make the city less susceptible to cannon fire.
The entrance to the tunnel is located next to the Cathedral of St. Vitus and it stretches below the Old Town to the Dolac Primary School. The 330-m long tunnel, which was dug into bedrock from 1939 to 1942 by the Italian military in order to protect civilians from aerial bombings, descends to a depth of 10 m in several places. It is 4 m wide and 2.5 m high on average. The main tunnel bore branches off in two directions, one leading towards the old city hall building and the other to the Cathedral of St. Vitus.
http://www.visitrijeka.eu/What_To_See/Attractions/The_Rijeka_Tunnel