Hey, smart people.

Joe here.

This is our planet.

And this is our moon.

This is how far apart they are to scale.

You might not know this, but compared to other moons in our solar system, our moon is weird.

It's 1/80 of Earth's mass, which may not sound like a lot, but it's ridiculously big compared to the size of the planet it orbits.

For comparison, Saturn's largest moon Titan is this big compared to our moon, but less than 1/4,000 of its host planet's mass.

So how the heck did this chubby little nugget, the moon we call "the moon," form?

The answer to that question is locked inside a vault at NASA's Johnson Space Center, right down the road in Houston, Texas.

And I recently paid them a visit and, in the process, got closer to the moon than I ever thought possible.

Does anybody have the combination to this thing?

This looks like a vault.

It turns out it is a vault.

It is, you know.

So this is a US Federal Reserve Bank vault from 1978.

Really?

That's pretty cool.

What's inside is worth more than money.

There is no price on what's inside this.

70% of the moon rocks on Earth are inside this vault.

70% of the moon rocks in there.

That is a very important room.

Yes.

Can we go in?

We can.

You want to open the door?

Yes.

I want open the door.

You turn that counterclockwise.

That's it.

Oh, this is so cool.

You should have felt that.

This feels important.

This weighs as much as a truck.

JOE: That door weighs more than 4,000 kilograms.

And behind it, are more than 300 kilograms of rocks brought back to Earth by the astronauts on six of the Apollo missions.

Back in the 1960s, even though scientists had figured out enough science to put people on the moon, they still didn't know how it had formed.

The rocks we brought back were the key to unlocking this centuries-long mystery.

Some scientists, like Charles Darwin's son George, had thought maybe the Earth once spun so fast that a chunk just ripped off.

Others thought the moon formed elsewhere in the solar system and was captured by Earth's gravity.

Most of these original theories sound pretty loony.

Incidentally, the words "loony" or "lunacy" come from the old idea that the moon made people temporarily insane.

But then in 1969, during the very first moonwalk, one astronaut made a lucky decision that gave scientists the hint they'd been waiting for.

In July, 1969, NASA blasted three men to the moon aboard Apollo 11.

And on July 20, Neil Armstrong and Buzz Aldrin touched down in the Sea of Tranquility.

They took some small steps, a few giant leaps, and a lot of pictures.

And then, at the last minute, right before they launched back to Earth, this happened.

These come from Apollo 11, right?

RYAN ZEIGLER: Yep.

This was the last sample collected on Apollo 11.

As Neil Armstrong's outside the LEM, the Lunar Excursion Module, getting ready to seal up the rock box, he looked inside and thought, it looks kind of empty.

And so what he decided to do was just shovel four shovels of dirt into the rock box and then sealed it up.

And it turned out being this sample.

And it was the largest single sample brought back from Apollo 11.

JOE: And that became a really important sample because, as people looked through it, they found little fragments of white rock.

And they're like, "Oh, it's anorthosite."

That's weird.

Why would there be anorthosite on the moon?

Anorthosite on the moon.

Wait.

Why is that weird?

You know what?

To understand why this scoop of soil was such a big deal, we need to back this story up a little bit to 1609 in Florence, Italy, where a guy named Galileo Galilei he had taken a new invention called a perspicillum and aimed it at the moon.

Today we call that invention a telescope.

Across the terminator, the line of light and shadow stretching across the partial moon, Galileo saw it had areas of high and low terrain.

He thought that, like on Earth, the darker, low-lying areas were seas.

So he called them "maria."

Apollo 11 touched down in one of these, the Sea of Tranquility.

The moon's highlands, on the other hand, were lighter in color like this spot where Apollo 16 landed.

And many of the shadowy shapes that Galileo saw we later realized where impact craters.

All of this together told us that the moon was really, really old.

And it had gotten its butt kicked for a really long time.

After billions of years of impacts with space rocks, the outer 5 to 15 meters of the moon's surface had been ground up into fine grained stuff called regolith.

And that's the sandy stuff that Neil Armstrong scooped up.

Back on Earth, scientists painstakingly counted 1,676 individual grains of regolith in Neil's shovelful.

And in that, they found 84 specks of white stuff they didn't expect to find there.

This sent NASA scientists on a Sherlock Holmes-style hunt for an explanation.

For that white anorthosite to get to the Sea of Tranquility, it must have been blasted there by a giant impact, like a meteorite, from hundreds of kilometers away in the moon's highlands, the lighter areas.

And if the highlands, which cover most of the moon, were anorthosite, that meant most of the moon was covered in that white rock.

Elementary.

RYAN ZEIGLER: To make anorthosite, to make the mean rock in the moon be this white anorthosite, you need to have a global ocean of magma that covers the entire moon, that's hundreds, if not thousands, of kilometers thick.

JOE: Let's have a little rock talk.

From studying Earth, we know anorthosite forms in a very special way.

It's an igneous rock, meaning it forms from cooling lava or magma.

As magma cools and crystallizes, anorthosite floats to the top because it's lighter and less dense than other stuff in the magma, like how ice solidifies out of liquid water.

It floats because it's less dense.

So for the moon's surface to be covered in this white stuff, the moon must have melted at some point, like the whole thing.

A moon that was all magma.

A magmoon.

But now scientists had a different mystery to solve.

How the heck do you melt a moon?

A guy named John Wood came up with this crazy idea that the moon formed through a giant impact, all based on his millimeter-sized fragments from one spot on the moon.

And everyone's like, no.

This is called the giant impact hypothesis.

Earth gets smashed by another planet-sized thing.

The giant melted cloud of debris condenses into our moon, creating a 1,000-kilometer- thick ocean of magma.

That's a pretty loony story too.

But none of those other older theories about the moon's formation could explain all of its weirdness, like why the moon's core is so small and so light compared to Earth's core, or why the moon has been slowly moving farther away from Earth for billions of years, or why the Earth and moon are made of the same elements and atomic isotopes.

The giant impact theory could explain all of these.

But to convince people that something collided with the Earth 4 and 1/2 billion years ago and created a magmoon, scientists needed a little more evidence.

JAMES IRWIN: Oh, man.

Look at that.

Almost see twinning in there.

DAVID SCOTT: Guess what we just found.

Guess what we just found.

I think we found what we came for.

JAMES IRWIN: Crystalline rock, huh?

DAVID SCOTT: Yes, sir.

You better believe it.

Look at the plage in there.

Almost all plage.

As a matter of fact, oh, boy, I think we might have ourselves something close to anorthosite.

RYAN ZEIGLER: This is what we call the genesis rock.

It's 15415.

It was collected on the Apollo 15 mission.

And the Apollo 15 site was selected, in part, to find this rock.

When they got it back and they did the studies on it, this was the rock that allowed scientists to unlock how the moon formed.

A body the size of Mars slammed into the Earth just after the Earth formed.

A ring of debris formed around the Earth, sort of like Saturn's rings.

Unlike Saturn's rings, that all collected into our moon.

So all the individual pieces collided together and formed our moon.

Every impact that brought the different pieces together was kinetic energy; it produced heat.

And so you melted the outer hundreds, and probably 1,000 kilometers.

There is an ocean of magma 1,000 kilometers deep.

So now you've got this giant body of magma, lava.

And as it cools, things start to crystallize.

And the heavy minerals would sink out.

And the light minerals would float.

And it turns out this white mineral plagioclase is less dense and it floated the surface.

So you have rockbergs floating in an ocean of magma.

And eventually, you have nothing but rockbergs.

And the entire surface is covered with this anorthosite.

And then you have billions of years of impacts.

And things get smashed and moved around.

But this is the original piece.

It showed us how things formed.

Anorthosites like the genesis rock turn out to be the oldest rocks on the moon.

They date from the earliest age of the solar system, from the time when Earth and the other planets were just forming.

And thanks to us putting a few humans on the moon to collect rocks, scientists were able to finally write the story of how the moon was created.

That story is a pretty violent one.

But I think behind this story there's something maybe even more significant.

The journey to the moon wasn't all about science at first.

It began as a race between the U.S. and the Soviet Union to build technology and to prove their military supremacy to each other and to the world.

But moon rocks helped change that.

We began to see space as a place for science that the whole world could use.

By the end of the Apollo era, we weren't just sending Air Force rocket jockeys up there.

We were sending scientists.

I think that changed NASA's mission forever.

The last man on the moon, Gene Cernan, summed this idea up beautifully just before humans left the lunar surface for the last time.

GENE CERNAN: And as we leave the moon at Taurus-Littrow, we leave as we came and, God willing, as we shall return, with peace and hope for all mankind.

JOE: Despite their old age, we're still learning new things from the moon rocks.

The history of the moon isn't wrapped up just yet.

Maybe we'll go back one day and science that chubby little nugget a bit more.

This has been absolutely incredible.

My mind has been blown, not only about the history of the moon but just how amazing is the science that NASA's been doing.

Thank you so much for letting us in here.

Thanks for the awesome outfit.

Absolutely.

Stay curious.

Um, so do I get to, like, take a sample home?

Oh, yeah, they didn't give you yours already?

Oh, wait.

No.