@smatjaz, hello!
Yes, some things around this (former) water pipeline are known to me. Maybe I don't have some things quite right, if someone knows more about this topic, they are always welcome and encouraged to correct me.
The Buzet - Brest - Žbevnica - Velika vrata - Golac - Obrov water pipeline was built after the First World War, when this area belonged to Italy (previously to Austria-Hungary). The main reason for its construction was definitely military in nature. It is known that this area is extremely waterless, and the army's need for water was extremely great at the time near the Rapallo border. At that time, besides the mentioned water pipeline, numerous military roads were also being built, which are still useful today (among others, the connection Vodice - Dane - Velika vrata - Golac). Well, let's return to the construction of the aforementioned water pipeline. In Buzet (by the Mirna river), a water pumping station was built, which still operates today, modernized, under the name "Istrski vodovodi". Here, at 80m above sea level, is the start of this water pipeline. In the pumping station, powerful pumps were installed that pushed water past the settlement of Brest (681m) through the slope of the 1014m high Žbevnica (here on the northern side, exactly such an object is preserved as at Velika vrata; the mountain trail to the top of Žbevnica passes by it). This object also served as a pressure relief and reserve water basin. From there, the pumps pushed water past the villages Dane (598m) and Vodice (651m) to Velika vrata (760m), where that underground water reservoir you visited is located. Further on, the water flowed to the village Golac (644m) and in Obrov (577m) joined the water pipeline that still runs through Matarsko podolje. Otherwise, quite a few data can be found on the web, where the history of the Istrski vodovod Buzet is published. The route of the aforementioned water pipeline can be nicely followed. If we drive from Obrov, past Golac and through Velika vrata to Dane and past Vodice towards Brest, and further in the direction of Rakitovec. The route does not go "over hills and valleys", but is sensibly routed through noticeable embankments and of course underground. Along the way, we can also see concrete pillars that served for filling water into truck tankers and, of course, also preserved concrete troughs for watering horses. I hope I have revived and recalled something small in memory of those distant times.
LP