On Isla Nublar, there was a large system of underground tunnels which were used for maintenance vehicles such as transport trucks and forklifts. They led to various locations on the island, and were used to transport food for the animals, and possibly the animals themselves. There were different sets of tunnels, all of which had different sizes. Adjacent to the larger transport tunnels were smaller ones where employees could actually walk. They had access to the larger transport tunnels via side doors. The third type of tunnels were more for technical stuff such as wiring and piping.
The tunnels led to the different locations of Jurassic Park, including an entry/exit point at Dr. Sorkin’s Parasaurolophus research pen, the Geothermal Power Plant, the Marine Facility, and an exit point at one of the many vantage points in the park. It is likely that the tunnels also led to underneath the Tyrannosaurus Paddock, thus explaining why the animal’s food came from underground.
After the Isla Nublar Incident, the group of survivors left behind spend a large amount of time navigating through them, and getting to know each other in the process. Gerry Harding, his daughter Jess, and Dr. Laura Sorkin enter them after a too-close-for-comfort encounter with Velociraptors at the Parasaur enclosure. They begin to make their way to the Marine Facility in search of phones to contact the mainland and get them rescued. Not long after, the raptors enter into the side walking tunnels, and stalk the group as they travel. Oscar sneaks his way around the remaining raptors at Sorkin’s research pen and enters the walking tunnels as well, and tracks the ones who animals that entered earlier. Soon enough, Nima and Yoder enter as well, after escaping from the Tyrannosaurus. Oscar fights a raptor and defeats it surprisingly. After time passes, Jess steals Sorkin’s pack of cigarettes and goes through one of the side doors to the walking tunnel to break away from the other two. When she goes to light the smoke, she is met face to face with a raptor, and barely escapes. The raptors get into the larger tunnel where the rest of the group is, but luckily Oscar shows up and scares them off, much to Sorkin’s surprise. The now reunited group travels to the Geothermal plant to fix the ventilation system, before being pushed into the third set of tunnels by the raptors, where they are then attacked by the Troodons. They run from the strange looking predators, and then are met with a puzzle of finding the right way to go. After more running from the Troodons, the group is split up again, with Gerry and Nima escaping up a ladder and coming out on the surface, and the rest of the group still going towards the Marine Facility.