The Nuka-World access tunnels were used by the personnel of Nuka-World, with the intention to move around unseen beneath the amusement park.
After Nuka-World was conquered by raiders, the access tunnels were used to construct the majority of "The Gauntlet," an enclosed system of deadly challenges for all newcomers entering Nuka-World via the Nuka-Express, built for the raiders' entertainment.
Upon entering the access tunnels, there is a Novice locked terminal to the left that allows the turrets to be shut down in the next room. These turrets awake only if the clapping monkey trap is triggered and can be avoided if shot from the other end of the room before being alerted. The exit door key is located behind the monkey, near the Nuka-World storage room key.
The next room consists Power Substation D, a generator room transformed into a small maze featuring a variety of traps and a cage with two rad-rat pups, an animatronic alien, and mines. Private Tyler's corpse can be found in a corner of this room.
The next door leads to the tunnels themselves, providing transport to other parts of the park before the war. Now it is filled frag mines and a truck bed full of Galactron Protectrons. Several waymarking signs are found here, indicating stops for each park route. Booby-trapped cars with mines underneath can start large chain explosions in the tunnel.
Further on, the tunnel has collapsed into a large cavern with a fragile plank walkway, followed by an Expert locked door. Falling down below results in several mirelurks rising from their hiding places. The deceased super mutant Crag is found in the mirelurk-infested waters below the bridge. A cave to the left allows access to the next area if one falls into the water. A fusion core can be found just after passing the wooden bridge.
Once this area is passed, the next obstacle is the booby-trapped door to Utility Access 8-N. The path following the door is likewise booby-trapped. When exiting the hallway into this room, the maglock door will close and gas will begin seeping out from the ceiling. The gas will gradually damage the Sole Survivor unless they are wearing a hazmat suit. Sections of the room will also spawn radroaches, including at least one legendary radroach, which upon its demise, will spawn a small swarm of smaller radroaches. Using the terminal to open the security gates will shut off the gas.
The Nuka-World utility substation password for the terminal is on a holotape, on a desk in the room on the right as one enters the area. The key for the door containing the terminal is in a door handle in a caged section, west of the desk with the holotape. The remains of Sergeant Marcks are in this section which once passed, leads to the Nuka-World maintenance shed.