Ever since we learned of the possibility of subterranean passageways on the island, we've wondered how the DI could have created all the miles of conduits through the volcanic island rock, in the short time since they landed on the island. But after seeing Ben turn the FDW, I'm starting to think now that most of the tunnels were dug out by the ancient islanders and the DI was able to add onto the existing tunnels, with offshoots to their buried stations. A few of the points in the show that make me think this way are:
1) The limited amount of time the DIs had to dig these tunnels. It seems as if they were mostly ready as soon as the stations were finished. The DI would have to be careful drilling around underground, in case there were still any "live" lava veins in the area. Wouldn't it be dangerous to be drilling everywhere quickly without careful and cautious geological exploration of a small volcanic island ahead of time?
2) We saw the ancient looking construction and glyphs in the cave/tunnel where Ben called on Smokey, and into where he had to crawl to get to the FDW. So some sub-structures were there before the DI arrived and set up shop.
3) If there were ancient inhabitants, there's a volcano, and they could have known about any empty dead lava tubes to use for tunnels. This could have assisted them in creating the vast system of conduits without modern technology -- unless they had some kind of Alien/Atlantean equipment to use of course (j/k, sort of). Otherwise, they could have just taken their time and primitively dug their way around over the course of many decades or centuries, since it looks like someone's been on the island for a very long time.
4) The Others/Hostiles seemed to pop up in the most obscure (from the stations and barracks) positions when they began to take the 815ers in the first two seasons. Why would the DI bother to scrape out tunnels in remote areas of the jungle and beach that weren't connected with their stations or experiments? The remote surprise access probably also aided the Hostiles in their attacks on the DI.
So, it looks as though there could have been a pre-existing tunnel system left from the early days of the island. After the DI arrived, the stations then could have been considered according to where the main tunnels were, allowing them to easily create new linking passages for the stations. Radzinsky's map may also show where the DI attempted to cut off access from their stations' tunnels to the Hostiles' tunnels after trouble started. Chances are, the DI never got to finish exploring all the subterranean tubes within the island before their situation began to deteriorate.