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Episode 8: The Hollow Earth

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Recap: The idea of a hollow planet - specifically Earth - has been around for a long time. And though it's rare, there are people who still believe Earth and the other planets and moons are hollow. I go through some of these ideas and give you no less than four independent methods to determine that Earth is indeed solid.

Puzzler: Let's say that you have a solid shell with compressible material inside it. You are able to rotate this shell very quickly about an axis through it. Does a cavity form? And if it does, what kind of shape would that cavity take on, and why?

Solution to Episode 6's Puzzler: The solution is somewhat abstract and I wasn't actually sure if anyone would get this. The concept is that of retrograde and prograde motion. Planets generally travel West to East relative to the stars in what is called "retrograde" because it "goes against" the normal way celestial objects move. But, due to the way orbits work out when viewed from Earth, there are times when the planets appear to stop in the sky and change directions, moving with the stars, East to West. This is called "prograde" motion. It's this switching directions that the astrologer in question was referring to. It's also not that rare that Venus switches, but since constellations occupy a relatively small part of the sky, switching from retrograde to prograde or vice-versa when it's in Orion isn't that common. (Here's a link explaining this kind of motion.)

Additional Materials:


  • I incorrectly stated that the difference between Earth's equatorial and polar radii is 14 km. The correct number is 21.3 km, or about 13 miles.

Transcript of the Main Material:


[Coast to Coast AM clip, July 26, 2006, Hour 2, ~1 minute in]

The actual history into the idea of a hollow Earth goes back at least several centuries. Of course one piece of iron-clad "evidence" comes from the Christian Bible, in which people have found passages that they say indicates Earth is hollow. I think this interpretation came after people found passages indicating it was flat.

Moving on, one of the first actual science people who thought Earth was hollow was the fairly famous astronomer Edmund Halley. He's that guy who has that comet named after him. In 1692, he proposed several shells around a solid center was what the interior looked like. His reasoning was that by this point we knew Earth's magnetic field changed over the course of human timescales (more on that in a future episode) and so he proposed that each interior shell had its own magnetic field and they were all moving independently. Where they overlapped or added or subtracted was what made the surface field change. For the time, this was not an unreasonable idea because we really didn't know much of anything about planet formation, magnetic fields, how much Earth weighed, tectonic theory, or anything else.

The next major contributor was John Cleves Symmes, Jr., an American army officer. No science training. But he believed Earth was a hollow shell 1300 km thick with holes 2300 km across at the poles, with more shells inside. Symmes somehow was one of the most famous of the early proponents, convincing even US president John Q. Adams to let him mount an expedition to search for the opening at the north pole.

Since then, there have been many others who have proposed the idea, perhaps fueled by French author's Jules Verne's "A Journey to the Center of the Earth," written in 1864. I actually read that book when I was youngER, though I was fairly disappointed they didn't actually make it to the center.

Anyway, the idea has persisted in movies, comics, books, and ideas for well over a century and a half now, though most take it in jest much like many don't believe Harry Potter was a leak by J.K. Rowling, permitted by the Illuminati to start acclimating us to that idea for their big reveal.

But some do. This podcast is about them, or at least some of them. It's also about Earth, not the other planets, since many also believe that other planets and moons are also hollow.


First, I'm going to talk about Symmes' idea of there being a hole at the poles and some modern ideas about that. [Coast to Coast AM clip, February 16, 2007, Hour 1, starting 15:41 in]

The main person talking in that Coast to Coast AM clip was Brooks Agnew, someone that I first mentioned in my special episode on Comet Elenin. For background on Agnew - no relation to the former VP - he has been saying for at least 5 years that he's going to go on this expedition to the North Pole to find the opening. Still hasn't happened. Originally was going to be free, now it's over $20,000 a ticket.

Anyway, there's really not too much to say about Agnew's claim of a hole. It's not there. And his idea that flying in an airplane makes you not able to see something that'a s minimum of 80 km across - or over 2x the size of Manhattan - is just ignorant. The Russians back in 2007 put their flag on the ocean floor at the North Pole. We have good satellite coverage for climate change monitoring, and there is no hole. It's just not there.

But maybe that's because satellites don't have a consciousness? ... [Coast to Coast AM clip, February 16, 2007, Hour 1, starting 22:23 in]

Earthquake Data

One of the only forms of actual observational evidence that pretty much all hollow Earth proponents point to is earthquake data. [Coast to Coast AM clip, December 23, 2004, Hour 1, starting 24:55 in]

Everything that Rodney Cluff just said is true, except the part about it being hollow. There are two types of waves generated from earthquakes that travel through Earth, called body waves. The first type is the "P wave" where "P" can be thought of as pressure, although the "P" stands for "Primary." These are the waves that arrive first, and they travel like a shock wave through material, compressing it in front of the wave and rarefying it behind the wave. The second type of wave is the aptly named "S wave" where "S" stands for "Secondary." S waves are transverse in nature, much like cracking a whip or waving a rope.

A good way to visualize these is with a slinky. Anyone who's listening who's younger than 25, let me know if slinkies were still cool when you were a kid. Anyway, take a slinky and put it on a flat surface so that one end is next to you and the other is away from you. Anchor down the other end so that the slinky is somewhat stretched out. Now, if you move your hand holding the end near you back and forth, that's an S wave. If you pull it towards you and push it away from you and keep that motion going, that's the P wave.

What's interesting about waves is that they'll travel in a straight line IF the density of the material it's traveling in is the same. Earth isn't. It has density differences and temperature differences and composition differences that alter how the waves travel. The effect is that they arc back to the surface as they go through a layer of Earth, they have a sharp bend in direction when they pass through a boundary like going from crust to mantle or mantle to core, and they can also reflect off the boundaries.

What's key in this point that Rodney Cluff misinterprets, besides earthquakes being used to show that Earth has a solid structure, is that the S waves can't travel through liquid, but P waves can. So the S waves can't travel through the molten outer core. P waves CAN and DO. But they wouldn't be able to travel through air. So this is actually evidence Earth is NOT hollow.

This is related to but different from the earthquake shadow zones. The shadow zone for S waves covers around 40% of the planet centered on the opposite side from where the earthquake originates. This is because the quakes can't travel through the outer molten core, but they do bend a little as they travel through the crust and mantle so it's not 50% of the planet that's blocked. The shadow zone for P waves is more complicated and is due to the refraction - bending - of them as they travel through Earth.

So earthquakes are actually a very GOOD way to figure out Earth is NOT hollow, and geoseismologists are actually able to create a pretty good map of the structure of Earth's interior from seismology data spanning over 100 years from nearly everywhere on the planet.


There's another hollow Earth claim to go over: Kevin and Matthew Taylor, from the land down under - as in Australia, not inside Earth - have an interesting idea of how Earth formed its hollow ... [Coast to Coast AM clip, November 24, 2005, Hour 2, starting 6:40 in]

This is an ... intriguing ... idea. But it lacks much basis in any form of reality. When planets form, yes, they "accrete" or gather material. Material is drawn to a forming planet due to its growing gravitational pull. Gravity works by pulling things together, not apart. As more and more material piles on, gravity pulls it down, together. As in not making a top surface that's denser and so has more gravity that starts to pull things from the center outward. This is really fairly basic physics that they just get wrong. And by "basic," I mean high school -level physics. Or at the least introductory Physics 101 in college.

But they don't seem to understand that because they claim ... [Coast to Coast AM clip, November 24, 2005, Hour 3, starting 36:31 in]

Independent Ways to Show Earth is Solid

That's a good transition into the second half of this episode because there are actually AT LEAST FOUR completely independent ways to show that Earth is in fact not hollow, where by "independent" I mean that you're not just debunking some crazy claim.

Earthquake Data: The first is earthquake data which I touched on earlier in the episode so I won't discuss now, but it deserves enumeration in this list.

Density: My favorite, somewhat easiest method that I think is most understandable deals with the density of material. To calculate the density of an object, you need to know its volume and its mass, then you just divide mass by volume.

We've known Earth's volume - assuming it's a sphere - for over 2000 years. Yes, we've known Earth was round and had a good estimate for its radius since the days of Eratosthenes, 2200 years ago.

But how do you weigh something you're standing on? It's like trying to weigh your bathroom scale with itself ... it doesn't really work. The theoretical idea came from that scientist's scientist, Isaac Newton, with his Theory of Gravity that states the gravitational force between two objects is the product of the two masses divided by the square of the distance. Fairly straight-forward. Problem is that there's also a nasty Gravitational Constant ("big G" as we call it) that you have to multiply all that by.

It was with Newton's Theory of Gravity that we get Newton's form of Kepler's Third Law - this Kepler guy seems to come up a lot in my podcast. Kepler's Third Law, or simply "K3" as we often abbreviate it, is that the square of the time it takes an object to orbit something is proportional to the cube of the distance it orbits. P^2 = a^3.

NEWTON'S form of K3 is more complicated. Divide both sides by a^3 of the proportion, and you have the period-squared over the distance-cubed is proportional to 1. Newton got rid of the proportion and figured out exactly what it's equal to: 4·π^2/(M*G), where M is the more massive object's mass.

I've now spit out a lot of equations. Remember - we want to know how much mass is in Earth. We have equations that could give us the mass of Earth if we know how fast something orbits around it and how far away it is. The obvious choice is the Moon. We've known how far away it is for centuries, and we've known how long it takes to orbit Earth for millennia. The only thing missing is Big G.

One of the problems is that gravity is by far the weakest of all forces in our dimensional space. It took until 1798 in an experiment conducted by Lord Cavendish before we had any sort of measurement of its value. It's actually still the LEAST-well-known value in all of physics constants today, measured only down to 4 decimal points with an uncertainty in the last two.

Anyway, once we had an idea for how big Big G was, we could then do the math and figure out how massive Earth is. It's a lot. We have its mass, we have it's volume - both well known by 1800, over 200 years ago. Simple division shows that Earth's density averages about 5.5 grams per cubic centimeter. For reference, water is 1 (it's actually defined as 1 -- or rather, grams and centimeters are defined from the density of water).

A density of 5.5 is actually fairly high - it's the densest of all the planets, with only Mercury coming close, and then Venus. We know there's a bunch of water on Earth, we can measure the density of rocks on the surface and the ocean floor (surface is about 2.7 gms, ocean is around 2.9-3.3), and so we know that deep down there must be some stuff that's much denser in order to make the AVERAGE density 5.5. We think it's a combination of nickel and iron.

Now let's get to the point: What would the density be if Earth were hollow? Yes, for those of you who forgot, that's what the point of this second of four points was for why Earth can't be hollow.

None of the people I've read or listened to actually have an estimate for the thickness of the shell, but they all agree it's air on the inside. Estimates I've heard range from 300 km to about 1400 km. Basic geometry allows the calculation, then, of the volume of a shell with an outer radius the size of Earth and an inner radius an appropriate size to give that thickness.

Doing the math - and I'll put this in the shownotes - gives us a density from 10.5-40.9 grams per cubic centimeter. Remember, the density of the crust that makes the oceans is roughly 3. Iron is 7.9. Silicon - which makes up a lot of Earth - is 2.3. The densest natural elements are iridium and osmium, which are about 22.6. The absolute densest material is only about HALF the density required under the thinnest shell model. The thickest would still require HUGE deposits of these ultra-heavy elements, even though we know they make up a VERY small part of Earth (which is why they're called "Rare-Earth Elements").

So in order for Earth to be hollow, we need to throw out Gravity and K3. Or make up some special reason why they don't apply here. The reason they exist is because they DO work, and so it's with this density argument that I think is the simplest method to understand why Earth can't be hollow, at least not the way these people claim. And I know I said "simplest," but I went into excruciating detail with it. I'll be posting a PDF summary of the math in the shownotes.

The next two won't be as detailed ...

Volcanoes: That brings us to the third independent method for why Earth isn't hollow: Volcanism. The current models which are fully supported by field geology, mineralogy, theory, and earthquake data, are that we have two basic types of volcanism on Earth -- hotspot volcanism which is like the Hawai'ian volcanoes, and volcanism based on tectonic plates. It's the latter that wouldn't work in a hollow Earth model -- actually, neither would work, but it's the latter I'm going to discuss.

The plate type of volcanism is where one tectonic plate dives under another. The subducting plate travels hundreds to thousands of kilometers beneath the other plate, where temperatures are much higher and it starts to melt. Hot rock is less dense, and it rises and will erupt as a volcano. That is a GROSSLY simplified model, but it works for this and sorta makes up for the detail I went into with densities of Earth.

The problem is this wouldn't work with a hollow Earth. The subducting plate wouldn't be able to go far down enough to heat up to create the volcanoes. And earthquake data actually map out the boundary of subducting plates, so we know where they are to at least a few hundred kilometers.

Moment of Inertia: The final one is Earth's moment of inertia. I put this one last because it's the most difficult one to understand, and I had to go back to my classical mechanics textbook to read up on. Moment of inertia, like many things in physics, is defined by equations. Put into words, the basic idea is that the moment of inertia is an object's resistance to changes in its motion. For rotational moments of inertia, like, say, a spinning Earth, it's the resistance to changes in its spin. For the physicists who are in the audience who are yelling at me right now, I realize that this is not an exact definition, but it works at a conceptual level to get the idea across. And I am not going into tensor math in this podcast.

The moment of inertia for MANY different things can be derived mathematically, and we can also measure them in the laboratory. A rod has a certain moment of inertia. A disk has a different one. A cylindrical shell has a different one from a thick cylinder which is different from a solid cylinder. And, importantly for whether or not Earth is hollow, the moment of inertia for a thin shell sphere is different from that of a thick shelled sphere which is different from that of a solid sphere. The theoretical values are all for a UNIFORM mass distribution. So, there are subtle variations depending on the distribution of mass and the EXACT amount of ellipticity in the sphere, etc.

The moment of inertia for a hollow spherical shell is 2/3*m*r^2. For a solid sphere, it's 2/5*m*r^2, or 60% less. This is fairly different. Since the m*r^2 term is present in almost ALL formulations for the moment of inertia, it is usually expressed for spheres in terms of I/(m*r^2), basically normalizing the intertia in terms of mass and radius. In those terms, all that's left is that 2/3 for a shell and 2/5 for a solid sphere.

When you do this, and you experimentally measure Earth's moment of inertia, you get 0.3308. Pretty close to a solid sphere, in fact MUCH less than that for a spherical shell. The difference is because there are different densities of layers as you go through, we're not actually a perfectly homogenous solid sphere, and because we bulge a bit in various places, like the equatorial axis is about 14 km bigger than the polar.

In comparison with Earth, the moon's moment of inertia is 0.394 ... VERY close to a solid sphere, despite what we heard earlier in a Coast to Coast clip.

Going to Jupiter, just for fun, it's moment of inertia is 0.254, much less than Earth's. The reason is that the density differences throughout Jupiter are significantly larger than Earth's, and so this affects the value.

The bottom-line is that Earth's moment of inertia is very consistent with a solid sphere, but it is very far from what it should be if it were hollow.

But, I suppose that doesn't matter, because all of modern science is built on a shaky foundation, or at least that's what the pseudoscientists like to claim ... [Coast to Coast AM clip, November 24, 2005, Hour 2, starting 31:15 in]

Provide Your Comments:

Comments to date: 3. Page 1 of 1. Average Rating:

Andrew Johnson   Derbs, UK

3:52am on Saturday, March 24th, 2012 

Very small point - Edmund Halley is almost certainly pronounced HAL-ee (as in HAL - the computer in 2001). Some say, at the time, it may have been pronounced "Hall-ee" (as in a Dance Hall)

The "Haley" comes from Bill "Rock Around the Clock" Haley and his comets...

Also, planet formation is a mystery - no one has been able to observe it happen. Gravity certainly isn't as simple as physicists portray. It is claimed that is the force of gravity shapes the universe and we have things like "dark matter" and "dark energy" (which people infer is there, but you can't see it). However, the force of magnetism is many magnitudes greater than the force of gravity - but it is supposed to not play a part in the structure of the universe.

Certain experiments by people like Eugene Podkletnov and Wilbert Smith show that Gravity is not quite like we think it is. So, certain arguments about gravity which you think are "solid" may not be quite as most people think.

Related to the "Hollow Eart... read more »

Stuart   Boulder, CO

9:10am on Friday, October 21st, 2011

Hi Jon, and thanks for the feedback. To answer your question, one has to first settle on a model to explain why it's hollow. Some people say it's because it spun initially really fast to make the hollow (like the puzzler). Some say it's because gravity actually works by making the shell and the center of gravity is a few hundred km within the crust (like the father-son team in the C2C clips). Others say other things. So under the Taylor scenario (the latter one), material strength isn't an issue because of how gravity "works."

For the magnetic field, the explanation I've always heard from these folks is that there's some sort of plasma phenomenon that makes a sun-like thing in the center of the planet. With that apparently comes a somewhat dynamic magnetic field. It doesn't really make any sense, but, well, that's par for the course.

Jon Paynter   Millbrook, Ontario

12:45pm on Tuesday, October 18th, 2011 

Thanks for the great episode. I got a little lost during the equations, but you brought it back beautifully.

A question I have is this: What kind of material strength would be needed to maintain a hollow cavity, given the tremendous gravity of the Earth? Could you not disprove the hollow Earth people by showing that in order to maintain a sphere, the material strength needed would be much greater than is possible.

Also, if the Earth is hollow, where does the Earth's magnetic field come from? If it's from the solid Earth, why does it shift around constantly. Seems like a liquid metal core is the only answer to those questions.

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