Below Pula is an underground tunnel world stretching almost across entire Pula, known as Zerostrasse. Pula's underground tunnels were erected during the WW1 to provide a shelter for people in case of air raids. The underground tunnel system consisted of shelters, trenches, galleries and passages, as well as ammunition storages and communication passages. Each hill in the town's core was drilled and dug up, thus creating an extended tunnel network. The tunnels of Monte Zaro, Monte Ghiro and the Castel are the largest.
The Castle, used as barracks, observatory, storage and prison during the Austro-Hungarian rule, hides two tunnel storages below its slopes. One is below the fortress itself, while the other was located in the foot of the hill. It is about 400 meters long, and it was used primarily for establishing the connection with the town centre. In its centre is the single area, a meeting point of underground tunnels with entrances from four different sides underneath the Castle. Entrances were carefully planned to provide a regular air flow, which temperature is between 14 and 18 degrees Celsius throughout the year. The width of the passage is between 3 and 6 metres, while its height is 2.5 meters.
The underground tunnels below the Castle alone could take about 6000 people, while the capacity of all shelters in Pula was more than 50,000 people, the number almost equal to the number of town's inhabitants. Such a capacity was achieved as a result of the permanent expansion of the tunnel network under the Italian rule and the continuous building of the fallout shelters during the WW2. One part of the tunnel is used today for holding various cultural and social events, from exhibitions to parties.