This is a different experience than the Great Siege Tunnels, but it is also included in the price for the Nature Reserve. If you are walking down from the top of the Rock, as I was, this is basically right on the pathway down, and is a good second stop after stopping at the Great Siege Tunnels, which are higher up.
These tunnels are on a much grander scale than the Great Siege tunnels, and the technology behind them is amazing, as most of them were drilled out in a short time during WWII, for use as dormitories, offices, even hospitals and schools, all bomb-proof. As a Canadian, I was interested to find out that much of the work was done by Canadian hard rock miners, who were used to dealing with the machinery needed to bore these giant tunnels through hard rock such as the limestone of Gibraltar.
You'll even pass through an area of faulting - interesting from a geological point of view to see what a fault plane looks like up close. There is also a nice view over the Mediterranean sea from an observation balcony (but watch your head when you go through the tunnel to this viewpoint! It's probably the reason that they issue hardhats to you when you start the tour).
More could be done here, I think, in terms of displays of information, but even as it is, it is very interesting.