The tunnels stretching below the Kingdom contain all the necessary mechanical apparatus and other equipment necessary to operate the park. It is also where the radioactive mist comes from, as the sludge that accumulated in the tunnels is very useful for deterring visitors.
During the Great War, the employees of Nuka-World and a few visitors sought refuge from the radiation in these tunnels. Because of "The Plan" that was in place, the tunnels were stocked with plenty of food and seeds, water, tools and extra lumber by Oswald and Rachel. About a week later, Mitchel appeared with even more supplies, especially weapons, explosives and medicine.
For hundreds of years, the employees were able to exist on their supplies, and they slowly adapted and ghoulified from the ambient radiation. By 2287, only Oswald the Outrageous has survived with all of his mental faculties intact; all the other employees have gone feral, used by Oswald to drive visitors from Kiddie Kingdom.
These tunnels can be accessed in three ways: through the door in the Nuka-Racer tunnel, a door in the very northwestern section of Kiddie Kingdom, or through two doors flanking a rotating Nuka-Cola bottle ride; the first option leads directed to where Oswald is, the second option is a tunnel that leads to the refugee living quarters rather close to Oswald, and the third requires navigating through a half-flooded tunnel, several storage rooms and gardens, and the main refugee living quarter.
Once at the room where Oswald is talking to his feral ghoul, there is a pit of radioactive sludge with a catwalk that leads into the room. This room contains a terminal that turns off the radioactive mist sprayers.
Inaccessible from the rest of the employee tunnels is the reactor outflow, it contains the skeletal corpse of Kevin Bennell along side two Nuka-Cola Quantums. This section of pipe is only accessible from the Secure Beverageer Lab beneath the Nuka-Cola bottling plant once power is restored. On the southern wall of the pipe niche is a wall terminal locked Novice, on it is a remote door control program that unlocks the gate to the pump room.