The Timavo river draws the border between two different environments. The limestone area of the Carso and the lagoon of Monfalcone. The peculiarity of this river lays in its bed which runs underground for a long stretch..

It has its sources South of the mount Snežnik, West Slovenia and runs under the name of "Reka" an initial stretch of 37 km, at 5 km from Trieste it flows into the cave of Skočan and continues underground for about 40 km, coming to the light near S. Giovanni di Duino under the form of rich wells whose waters originate a single river just 2 km long, finally outflowing in the gulf of Trieste. It is Europe's shortest river but rather rich in waters. The unusual show of its foundering and its mysterious rise out of the rocks unchained the fantasy of men, pushing them to guess the most different explanations along the centuries.

The first one who made an hypothesis about this subject matter for long time unexplained was the naturalist Cluverius in 1610. He thought that there was only one river that, after having sunk, gives birth to three different mouths ending in the Adriatic sea. The mistery was cleared in the end by the father of the modern speleology Eugenio Boegan, at the end of the thirties. He noticed that the problem was much more complex.  In the Summer the Timavo gives back much more water than what the Reka poors in the Skočan caves..

The speleologist understood that there was not any direct link between Skočian and S.Giovanni in Tuba, but 37 Km of tunnels and caves linked to big catch basins that "administer" the water to be kept or given back to the Timavo depending on the period. It has been demonstrated that, under certain situations, a lower water input of the Reka is balanced by the resurgences of the Carso basin, from the Isonzo river and by the carsic lakes of Doberdò and Pietrarossa. Still today, notwithstanding the many explorations proving that its waters originate from more than 60 mt below the sea level, the sole windows where it can be seen are the bottom of the "abisso dei Serpenti" in Divaccia, the bottom of the abyss of Trebiciano and the "Pozzo dei Colombi".