Have you time-traveled from the early 18th century? If so, a hollow earth is still somewhat scientifically viable.
Unfortunately, since then, our knowledge has grown leaps and bounds. Two major advances stand out.
First, the measurement of the density of earth. Based on Newton’s theory of universal gravitation, we can compare the gravitational force due to known masses and the earth. Both the Schiehallion experiment and the Cavendish experiment showed that the mean density of the earth was much higher than the density of rocks on the surface. In fact, the earth is on average twice as dense as the crust. Far from being hollow, the earth had to have a denser interior.
So what if there is a very dense shell below the crust and it remains hollow inside that? Well, that idea can be tossed to the bin based on seismology. We can track how the different kinds of earthquake waves move inside the earth.
And based on this, it is possible to reconstruct the layers of density in the interior.
We know, for instance, the crazy fact that the outer core of the earth is liquid because S-waves do not travel through it. There is real marvel in the structure of earth, there is no need to imagine up hollow regions and advanced civilizations inside.
Sure, if you want to go all conspiracy theorist, you can claim that seismology is a sham and that Cavendish experiment is fake (no matter that it is a regular undergraduate level experiment, one that I have performed myself). Or maybe doubt gravitation itself like the flat earthers. But the rest of the world will carry on with their lives, using all these concepts in daily science and technology (earthquake monitoring, anyone?) and discarding the hollow earth in the loony bin where it belongs.
And the saddest part is that I did not have to use my physics PhD or even the knowledge from my undergraduate studies to answer this. Newton’s law of gravitation was ninth standard physics. Layers of the earth including the seismic reasoning were in the ninth or tenth standard geography textbook. The only reason someone would still parrot a hollow earth is if they utterly failed high school or if they have a paranoid level of distrust that should be considered clinically insane.
That, or they are a time traveler from the eighteenth century.
