<div class="pull-right"><img src="https://steemitimages.com/DQmTTZm3ChwNvRYGfu3TxPhGJ3xF7Emuf5P4Htk9NDjLfzc/image.png"><br><center><sub>C. Durand Chapman <a href="https://en.wikipedia.org/wiki/Hollow_Earth#/media/File:Map_of_the_Interior_World.png">link</a>
Public domain image.</sub></center></div>

The Hollow Earth theory is another one of those pseudo-scientific theories that does not have the good grace to die. This post will be another attempt to drive a stake through its stupid still-beating heart.

The concept is that our home planet is hollow and the interior has significant surface area. It has appeared in the folklore of many cultures over the centuries and is claimed to be the location for Atlantis and the source of flying saucers.

Let's go over the evidence as to why the Earth is not hollow and is in fact quite "un-hollow".

<div class="pull-left"><img src="https://steemitimages.com/DQmRNCju9UR6YA1brUyktpEioYMJTr32bdiJKX5TUHVYTk6/image.png"><br><center><sub>United States Geological Survey <a href="https://earthquake.usgs.gov/learn/glossary/?termID=170&alpha=S">link</a>
CC BY-SA 3.0 license</sub></center></div>

#### Seismic Wave Evidence
Earthquakes, volcanoes and even atomic testing set the ground to shaking, quite violently in some cases. Seismic stations are set up all around the planet to record the strength and times of any ground movements.

When an earthquake occurs, two types of earthquakes waves will emanate from that area called P-waves and S-waves.

P-type waves (Primary waves) are compressive waves that are longitudinal in nature. As the waves travel they compress and relax the ground below in succession as they travel through the Earth. The waves will travel at a characteristic velocity and their direction of travel will even diffract (change direction) as they encounter denser and denser material.

The most important part about a P-wave is that it can travel through liquids (unlike S-waves which I will get to in a bit). This important difference between P-waves and S-waves is that it has allowed us to deduce that the outer core of the Earth is a liquid (liquid nickel-iron in fact and it is very hot). 

S-type waves (secondary waves) are shear waves and are tranverse in nature. These waves are more like the movement on a guitar string (side-to-side). Due to this motion they cannot travel through liquids. When they encounter the liquid outer core they will simply bounce off of this interface and travel back up to the surface of the Earth. The signal from an S-wave can therefore be occluded by the liquid core of the Earth.

One could argue that S-waves would also reflect off of the interface of a hollow core but there is no arguing against the evidence for P-waves being able to travel right through the liquid centre of the Earth.

#### Limits To The Strength of The Rocks
The Earth is  a sphere (oblate spheroid to be exact) and it is that way because it has so much mass that it deforms the rocks into the least-energy shape which is a sphere. The Earth is said to be in [hydrostatic equilibrium](https://en.wikipedia.org/wiki/Hydrostatic_equilibrium#Planetary_geology).

If the Earth were hollow the material of this planet would have to be composed of some super-strong and as yet unknown and unobserved material to be able to hold up the incredible weight of the Earth's mantle, crust and continents.

#### Average Earth Density
The [average density of the Earth](https://en.wikipedia.org/wiki/Earth) has been calculated mostly from the equations for the orbits of the Moon and the artificial satellites. If you know the period of a satellite then you can figure out the mass of the object that it is orbiting from this equation:

<center>![](https://steemitimages.com/DQmRUsfP1QZQDRrjUb4RjbZTJ2gMprumHvuThmLY7SX2BVj/image.png)</center>

where,
T is the period of the orbit in seconds,
r is the distance from the Earth's centre in metres,
G is the Gravitational constant ( 6.67 x 10-11 m<sup>3</sup>/kg·s<sup>2</sup>.) and
M is the mass of the Earth in kilograms.

We know the diameter and therefore the volume of the Earth. From these we have worked out that the average density of the Earth is 5.514 g/cm<sup>3</sup>.

If the Earth were hollow then its density would be much lower than this value and indeed the orbits of both the artificial satellites and the Moon would be greatly affected.

<div class="pull-right"><img src="https://steemitimages.com/DQmR7v4z9Q9pVGHz9Y6Wa5zogEEJcUfSCgcrJBtfN4LJghH/image.png"><br><center><sub>Bkilli1 <a href="https://en.wikipedia.org/wiki/Geothermal_gradient#/media/File:Temperature_schematic_of_inner_Earth.jpg">link</a>
CC BY-SA 3.0 license</sub></center></div>

#### Heat
Deep mining has been an industry for a long time now and miners have learned that the deeper you go the hotter it gets. 

The term for this is [geothermal gradient.](https://en.wikipedia.org/wiki/Geothermal_gradient) As you go down the temperature increases by about 25 °C to 30 °C per kilometre. Eventually the heat becomes a significant limitation in how far down that you can mine.

The heat is due to the energy of impact from the original formation of the Earth and the decay energy from radioactive elements.

If the Earth were hollow then there would be a very different heat gradient (or none at all). A hollow Earth implies it was constructed or hollowed out. Most of the heat source would have been removed or not have existed in the first place.

#### Closing Words
So the hollow Earth theory has been thoroughly debunked over the years and there is substantial evidence against it.

Most importantly we don't need a hollow Earth theory to explain the origin of flying saucers as it is well known that the [Moon Nazis](https://en.wikipedia.org/wiki/Iron_Sky) are the cause of this phenomenon.

(When in doubt always blame the Moon Nazis.)

*Thank you for reading my post.*
#### Post Sources
https://en.wikipedia.org/wiki/Hollow_Earth
https://en.wikipedia.org/wiki/Seismic_wave
https://en.wikipedia.org/wiki/Earth
https://en.wikipedia.org/wiki/Geothermal_gradient
https://en.wikipedia.org/wiki/Hydrostatic_equilibrium#Planetary_geology
https://en.wikipedia.org/wiki/Iron_Sky