If the Earth were hollow, we’d all be dead, right? No magnetic field? Much likelier that the Earth would have enormous cavern areas embedded in the crust
Hm. Caverns.
DUMBs, I hear they are called.
Deep Underground Military Bases.
Filled with Reptilians.
Most clearly, as you suggest, there may be UFO bases underground or underwater. More speculatively, at least some UFOs may be from alternate dimensions that are coextensive with Earth and the solar system. And some UFOs may even be from a future human-derived civilization that sends researchers back in time to investigate and improve their origins. Notably, these possibilities are not mutually exclusive, nor do they negate the ET hypothesis. Such a multiplicity of theories makes the simplicity of the Hollow Earth almost attractive in comparison.
As esteemed French UFO researcher Jacques Vallée has noted, the entire subject of UFOs is obscured by misdirection and deception from both UFOs and human authorities. We may be approaching an era of clarification though. Supposedly, humanity (meaning the US) is now possessed of a rudimentary but effective self-defense capability, which is leading to a gradual process of disclosure so that we may better protect our interests. Who knows, but we may soon regard "ET go home" as a command to meddlesome vistors instead of as just a movie line.