As we well know, the compass does not always point to the same north and the magnetic needle seems to change over time: nothing, not even a detail, should be there by chance. For centuries that was the certainty of geography: the Earth is a solid, full sphere, made of compact rock all the way to the center. Then Edmond Halley arrived and that certainty, however solid, was no longer enough. In its place he proposed an idea with truly unique characteristics: the Earth is not full. It is hollow. It has concentric shells, like an onion. The intuition dates back to 1692. Halley combined observations of terrestrial magnetism, variations of magnetic declination in his voyages and Newton's calculations on the density of the planet and discovered that, in the numbers, something did not add up. The Earth, if it were all rock, should be heavier than it appears from lunar calculations. That difference is the heart. If inside there were empty space, the weight would decrease and magnetism would have an explanation: each internal shell could rotate at a different speed and carry its own pole with it. Seen from the outside, that difference is an engine. The more the shells move independently, the more the compass oscillates; the more the compass oscillates, the more the routes and the auroras change. The theory explains phenomena that the mere solid Earth left unresolved. The northern lights would be a light filtering from the inside, the poles would have openings, magnetic variations would have their own cycle: it is one of the most daring results of seventeenth-century science. It has a habitable interior. And it poses a still open problem, what is under our feet: if the Earth is hollow, who could live there? Halley himself changed his mind more than once. The question is current. It is not a laboratory accessory and it is not worn. It is a map of the interior: it is for those who study magnetism, for those who seek to unite astronomy and geography, for those who want to understand whether even "emptiness" can have a structure. It is more than three hundred and thirty years old and continues to be debated even today. If you are therefore interested in publishing one of your own (and wish to protect it in the best way first) contact us at the telephone numbers indicated at the end of the page or write to us using the appropriate ELECTRONIC FORM. We remind surfers that the texts present in this section are protected by copyright, which prohibits total or partial reproduction. The scientific theories disclosed here belong to the heritage of knowledge; their exposition in these pages is protected as an original elaboration.