The Netherworld, aka the Netherrealm, is the home of all Ghosts.
The Netherworld exists below Pacopolis. As a normal Pac-Worlder, the only way to get there is through a portal. Ghosts mainly use portals to get to Netherworld as well, however in some episodes, it is shown that Ghosts may just fly through the ground to get from Pacopolis to the Netherworld, and vise versa. Due to its location, the Netherworld has uninhabitable conditions for anyone besides the ghosts, such as having barely any oxygen, having no natural lighting, having no delectable food, and having some of the worst smells in existence.
Aside from Ghosts, certain varieties of Monsters and other creatures such as Nether-Gargoyles, Nether-Slugs and Nether-Dragons call the Netherworld home. the Netherworld is also where Betrayus’s Castle, the Temple of Slime and Dr. Buttocks’s Lab are situated.
Pac-Worlders and most other species and creatures are unable to live in the Netherworld, due to there being no oxygen, but some other species such as Pointy Heads are seen to be able to breathe perfectly fine, despite there being no oxygen. The only way Normal Pac-Worlders are able to breathe in the Netherworld is by either eating a Power Berry, or by wearing a snorkel.