Billions of years ago,
another planet smashed into Earth.
An event of this epicness
could have created our Moon.
It also could have brought
alien life to our planet.
Four and a half billion years ago,
Theia, a protoplanet the size of Mars,
struck our young Earth.
This collision scattered pieces
of both planets into space.
And many scientists argue
that these pieces
eventually coalesced into our Moon.
But what happened to the rest of Theia
is still a mystery.
The existence of blobs of material inside
Earth's mantle,
known as low sheer velocity provinces,
could be evidence
that parts of Theia fused with Earth.
These chunks are up to 1,000 km (621 mi)
in height and several times that in width.
They sit below Africa
and the Pacific Ocean,
straddling Earth's core
like a pair of headphones.
But other theories,
some dating back centuries,
suggest a completely different
explanation of what exists
under the surface.
A lush tropical paradise for aliens.
The Earth consists of four layers.
The crust, the mantle
and the outer and inner core.
But there hasn’t been any
successful exploration below the crust.
So there is much we don’t know
about the realities below the surface.
This has led to spectacular speculation,
like the Hollow Earth theory.
According to this theory,
our planet could be
a series of nested spheres,
centered around a central core.
In between these shells,
atmospheres could exist
that are capable of supporting life.
In the 17th century,
Edmond Halley,
the scientist who discovered
Halley's Comet, backed this theory.
Many others have added to it,
suggesting a small Sun
could be hanging
in the center of the Earth.
And plants, animals and humans
could be living there.
And not just humans,
but a race of superhumans.
Immortals who possess
sophisticated technology
to build hundreds of subterranean cities.
But if there were
aliens inside the Earth,
what would the conditions for life be?
Could the inside of our planet
be a luscious paradise?
Or a hot, hellish dump?
If you were to visit one
of the low-sheer velocity provinces,
you'd have to go 2,900 km (1,800 mi)
below the surface.
Here, you'd be surrounded
by magma, or liquid rock,
with temperatures
as high as 3,700 °C (6,692 °F).
And the weight of the crust
and mantle above you
would increase the pressure to over
237,000 times
the atmospheric pressure,
you know, on the surface of the Earth.
Whatever life you find here
would need to exist
in these extreme conditions.
And these life forms would need
to evolve to extract oxygen
directly from the magma.
Or live without it.
They'd need to survive
without any sunlight.
Radiation would be
the primary energy source.
And lucky for them, the radioactive decay
of uranium, thorium and potassium
in the Earth's crust and mantle
already account for the primary source of heat
in the Earth's interior.
There could be all kinds of undiscovered
creatures down there.
But until you could
withstand the pressure
and heat to find out,
we'll never know.
And as for the Hollow Earth,
it's nice to imagine
a paradise world underneath our feet
instead of layers
upon layers of hot molten rock.
And who knows, maybe there is alien
life down there.
Just like all those underwater aliens
hiding in the world's deepest lake.
Oh, wait.
That's a story for another WHAT IF.