(swords clang)
(screaming)
(dramatic music)
- That's old Skeletor taking a dramatic fall
into the bottomless pit beneath Castle Grayskull
in 1987's "Masters of the Universe."
- So what if Earth had this same sort of chasm,
what would it be like to fall straight through it?
Let's explore, Earthling.
(upbeat techno theme music)
- [Man] The bottomless pit is always a fun bit of fantasy,
regardless if it's the bottomless pit
of the Bible's Book of Revelation,
or just the one beneath Castle Grayskull here.
I mean, this was a major feature of the franchise, too.
The original '80s He-Man cartoon
devoted an entire episode to it.
- [Woman] But what actually constitutes a bottomless pit?
What about a single vertical shaft that begins on one side
of a planet, and extends all the way out the other side?
On Earth, such a shaft would stretch a total of 7,900 miles,
or 12,700 kilometers.
Sounds like a bottomless pit to me.
- [Man] It may seem ridiculous,
but physicists love this thought experiment.
Yet like anything involving crazy tunnels through the Earth,
you have to suspend a bunch of basic realities.
Forget about Earth's outer liquid core of iron,
and just pretend the planet's a cold,
uniform, inert ball of rock.
- [Woman] Someone jumps in the hole, perhaps in Chile,
looking to emerge in China.
What happens? Well, you start falling.
Gravity pulls on us at 32 feet
or 9.8 meters per second squared.
That means that for each second you fall,
you speed up by 32 feet per second,
but only near Earth's surface.
- [Man] Now, gravity is a function of mass,
and mass is a property of matter.
When you're on the surface of the Earth,
all of the planet's matter is beneath your feet.
But as you fall through this imagined hole,
more and more of it surrounds you, exerting its own gravity.
- [Woman] These horizontal tugs cancel each other out,
but the increasing proportion of mass above your head
exerts a growing counter-force to the decreasing mass below.
So your crazy acceleration actually slows a bit
as you near the core.
- [Man] And when you reach the core,
or the section of the tunnel extending through it,
your acceleration due to gravity reaches zero.
The mass of an entire planet surrounds you,
canceling out gravity and leaving you weightless.
- [Woman] Oh, but you're not stopping here to float around.
You're going way too fast.
Halfway to the center your speed hits 15,000 miles per hour,
or 24,000 kilometers per hour.
- [Man] You reach the core segment of the tunnel
a mere 21 minutes after jumping in,
and you blast right through at 18,000 miles per hour,
or 29,000 kilometers per hour.
We're talking re-entry speeds, here.
- [Woman] Another 21 minutes pass,
with gravity slowing the whole way,
you reach the far side of the Earth
and stop briefly in midair.
This is where it pays to have a pen pal
on the other side of the world,
because, unless someone catches you, you'll fall back down
and enjoy the entire 42-minute, 12-second fall again,
and again, and again.
- [Man] In theory, you could keep falling
back and forth indefinitely,
falling forever in this never-ending bottomless pit.
Again, for this thought experiment to work,
you have to dismiss a load of other concerns,
including the Coriolis effect and angular momentum,
that would certainly smash you into the tunnel walls
on the way down, and back up.
- [Woman] Again, physicists love this thought experiment.
And in 2015, Alexander Klotz, a graduate student in physics
at McGill University in Montreal,
tackled the problem anew using seismic data
from the Preliminary Reference Earth Model.
Factoring in updated Earth density numbers,
Klotz's imagined fall through the planet
came in at 38 minutes and 11 seconds, a new record!
- [Man] Either way you shake it, 42 minutes or 38,
that's just enough time to watch two back-to-back episodes
of the old He-Man cartoon on your phone
as you fall through the Earth.
You just might want to skip the intros.
Whoa.
- This is a lot more relaxing than I thought it would be.
- Oh yeah. I can't wait 'til we hit the core,
for that weightlessness.
- It's gonna be awesome.
- Gigantic pits are awesome!
They combine all of my favorite things:
mystery, danger, attractive nuisance, and scary geology.
Here are some of the coolest.
- [Announcer] I always thought Earth was solid. But is it?
Some people don't think so.
In fact, Hollow Earth theorists think the Earth's surface
is only 800 to several thousand miles thick.
- Have you ever heard
that if you're in a free-falling elevator,
and you jump up right before the elevator hits the ground,
you can save your own life?
Well, that's probably not true.