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.