The Earth is almost eight thousands of miles in diameter. There’s no way that we can see through it, and even the deepest mineshaft ever drilled only penetrated 7.5 miles down.
“The Kola Superdeep Borehole was just 9 inches in diameter, but at 40,230 feet (12,262 meters) reigns as the deepest hole. It took almost 20 years to reach that 7.5-mile depth—only half the distance or less to the mantle. “
Ask Smithsonian: What’s the deepest hole ever dug?
Let’s visualize this
Clip from ‘Everything You Need to Know About Planet Earth’, by Kurzgesagt – In a Nutshell. (animation studio and YouTube channel)
So that is just than 0.19 percent of the way in. (Earth’s mean radius is 3,963 miles /6,378 kilometers.)
So how in the world can we know what is deep inside the Earth?
I. Listening to sound waves
A major way that we learn what is inside the earth has to do with sound. Think of ultrasound and sonar.
Sonar (SOund Navigation And Ranging) is a way to use sound to navigate, communicate with, or detect objects under water.
Medical ultrasound is a diagnostic imaging technique used to see inside the body.
Here’s a medical ultrasound of a human heart beating.
If only we could build a giant ultrasound scanner, then we could scan the Earth and see what is inside? Well, that might be kind of difficult. But maybe we don’t quite need to do that: ultrasound scanners create a powerful sound that goes in to a body, and then they listen to the sounds that are reflected.
Nature does half of this this job for us. Earth creates earthquakes all the time, and they make the Earth ring like a bell, if we listen for it.
So let’s build a system of sound detectors buried in the Earth to listen to these sound waves. Here’s an example of a traditional seismic detector.
Earth Science, Tarbuck & Lutgens, Chapter 8
With hundreds of these at locations all around the world, we can hear earthquake waves – seismic waves – traveling through the Earth.
Image from USGS, earthquake.usgs.gov/learn
With this information we can infer what is inside the Earth.
II. Experiments on minerals and rocks
We perform laboratory experiments on surface minerals and rocks. We place the under very high pressure and temperature, simulating what goes on inside the Earth.
This section TBA.
How we know what lies at the Earth’s core. BBC
Journey from the center of the Earth. Symmetry magazine.