Another "scientific proof" of the existence of the pyramids, as presented by the Foundation in this document (in Bosnian (bs)  and in English (en)), is the so-called "extensive tunnel network" in the valley. Mr. Osmanagic claims that this "tunnel network" is connecting all the "colossal objects" of the valley, and that the existence of this underground network is characteristic for the pyramids in all the world (this last idea is quite disputable, as a lot of pyramids do not have an associated underground complex).
This idea, of a tunnel network, is founded, it seems, on some old rumors running in Visoko, rumors that have been gathered on these webpages (en). There one can find this story about the tunnels:
"There are local stories of Bosnian children entering the smaller pyramid, just across a small river that divides the site, and emerging at the top of the larger pyramid, next to the ruins of a medieval walled town."
From these stories Mr. Osmanagic deduces the existence of this famous "underground complex", and even describes it quite precisely: there would be two lakes under the pyramid of the Sun; they are artificial tunnels, with straight walls, all connected with right angle crosses; the network would be several kilometers long, 3,800 meters, it is said, for the main tunnel which would connect the three pyramids themselves (see this press-release (bs) from the Foundation, where it is asserted that the tunnels connect the village of Ravne with the hills of Visocica, Pljesevica and Krstac).
What are really these tunnels? The exploration and mapping of this "underground complex" are part of the Foundation projects; especially, the young swedish archaeologist Sead Pilav was supposed to do the mapping of the tunnels. Unfortunately, he swiftly quit the project, apparently disgusted by the lack of professionalism on the excavations (see also here. However, there have been some explorations of a tunnel, the one whose entrance is at Ravne, about 3 km from Visocica. The explorers have been able to progress about 300 meters inside the tunnel, then were stopped by the collapsing of the ceiling (a really bad video of this first exploration can be seen here); no other tunnel has been, apparently, found or explored in 2006. So that we are very far from a confirmation of this famous "underground complex" that would connect Ravne with the three pyramids...
As for the question of the origin of this tunnel: natural, or artificial? It seems that the artificial origin cannot be disputed: the tunnel is delved, according to the published photographs, in a deep layer of Miocene conglomerates, conglomerates which are not very prone to dissolution:
- Le tunnel de Ravne
- Tunnel in Ravne - Source
What is more, it seems that ventilation shafts are made at regular intervals (15 to 30 meters); the analysis (bs) made by the Foundation team on the ventilation of the first 200 meters shows a quite good quality of the air, and a light draught.
So it is very probable that it is actually an artificial tunnel. Made by whom? when? for what use? There is no way, for the present, to answer these questions, as there is no scientific study and report published. One can however make a few remarks and hypothesis:
About the age of the tunnel, the only indication for the present is, at some places, the existence of stalactites 2 to 5 cm long; it’s difficult to deduce from that a precise age, without data on the stalactite growth in this region, but even with a very slow growth of 0,1 mm/year, that would mean at the best a few hundred years.
All the Zenica-Sarajevo region, where is Visoko, is a well-known mining basin; extraction of coal, as well as iron and copper, is attested there at least since the Roman time, and even the Bronze age for the copper.
Talking about rumors and local stories about these tunnels, I have found several mentions of the legend (bs) according to which Queen Katarina, last Queen of the Kingdom of Bosnia, escaped from her fortress of Visoki using and underground tunnel at the time of the Ottoman invasion of 1463...
I’m quite sure that, in France, in England, in Germany, as well as in Bosnia, each old ruined castle has its own legend of lost underground passages, and its stories of children supposed to have found / to have fallen / to have been lost in these underground ways. But there is quite a long way from these stories to the "proof" of the existence of an ancient and mysterious civilization...
 Most of the reports have disappeared from the Foundation site - whether deliberately, or simply by accident, it is not known; so that I join a copy below the article for the interested reader.