When I was a little kid, I used to love to watch the television series Superman. Even though I knew he was a fictional character, it was fun to fanaticize about his magic powers. I remember one episode, where the world's deepest oil well was drilled. One day, this "alien" crawled from the well from his home inside the Earth. I remember thinking, "I wonder what's down there? Could there be people living in the center?". Harking back to the old adage, The more things change, the more they stay the same', I came across a website the other day that was the product of the Hollow Earth Society. Yes, that's right. The Hollow Earth Society. They believe the center of the Earth is hollow, populated by humanoids, who make frequent trips to the surface in strange vehicles resembling spacecraft. There is a secret opening near the North Pole!
This is not the first time that strange ideas have surfaced (sorry for the pun!) about the Earth's interior. In the 1960's, a project was undertaken by the United States to drill a well, that for the first time penetrated the crust and into the mantle. The imaginary boundary between these two layers is called the Moho, named after a famous geologist Andrea Mohorovicic. They never made it of course, and the attempt was stopped in part due to fears that punching a hole through the Moho might have unexpected results such as an outpouring of lava. This of course is ridiculous, for the Moho is not some sort of trap door holding back all layers of the Earth below.
Like a doctor who uses X-rays to look inside a patient, seismologists use seismic waves to peer inside the Earth, without actually having to go there. The two main types of seismic waves are P and S waves. The speed of these waves is directly related to the density of the material through which they travel. This helps geologists determine the composition of the Earth's interior. P waves can travel through solid and liquid rock, while S waves cannot travel through liquids.
From the study of seismic waves, we now know that the outer core of the Earth consists of a liquid. Not water, but primarily liquid iron, whose swirling caused by the Earth spinning results in the Earth's magnetic field.
The inner core is like a pit in peach, made of solid iron and nickel.
The mantle is a little bit less mysterious, as there are areas on the Earth where mantle material has been brought to the surface, and consists of various igneous rocks.