ALI ROGIN: Between the 16th and 19th centuries, millions of captive Africans were forced onto slave ships and trafficked across the Atlantic Ocean.

As many as 1,000 of these ships sank on the journey, and fewer than 20 of them have been found and properly documented.

For our series Race Matters, I recently sat down with National Geographic explorer and writer Tara Roberts, who has been traveling the world documenting these underwater wrecks and the intrepid group of primarily black divers working to uncover them.

She tells these untold stories in her new memoir, "Written in the Waters."

Tara, thank you so much for joining us.

Your journey that you relay in this book took you across four continents, but you got the idea for the project that became the book here in Washington, DC.

Tell us about that.

TARA ROBERTS, Author, "Written in the Waters": This was in 2016, and I got offered tickets to go and visit the National Museum of African American History and Culture.

And it was there that my life completely changed.

I saw a picture of a group of primarily black women in wetsuits on a boat.

I'd never seen a picture of black women in wetsuits on a boat before.

So it really struck something in me.

I discovered that they were a part of this group called Diving with a Purpose, and that they spent their time searching for and documenting slave shipwrecks around the world.

And that just it floored me.

It made me want to be a part of that work some kind of way.

ALI ROGIN: When did you realize that your story and the histories that you were telling were all intertwined and could be part of this narrative?

TARA ROBERTS: It wasn't until I started to meet descendants of people who were on the ships that I started to think about my own ancestry, which is something that I didn't think about from the start, which maybe is a little surprising.

I mean, I am a black woman, and it is the slave trade, but I just didn't think about it personally.

And then traveling around the world and encountering so many different people from the African diaspora who had different understandings and perspectives of this history, it just had me question a lot of things.

ALI ROGIN: You began your journey talking about how you were kind of trepidatious about going down this road, but you end it in this place of empowerment.

Writing on the bottom of the ocean, I feel agency instead of sorrow.

How has this work empowered you?

TARA ROBERTS: I think that there's something about actually encountering real pieces of the past that makes the past clear and undeniable.

But then there's something about the fact that I'm taking on this work.

These other divers are taking on this work.

Historians, archaeologists, like all of these people are saying that we're not going to wait for anyone else to prioritize this history, that we are raising our hands and we're volunteering our time to bring this history back into memory.

Like, that is not sad work.

That is powerful work.

I can say to the ancestors that I see you, I honor you, I remember you.

I haven't forgotten.

And that is really something for me to feel empowered around.

ALI ROGIN: What were some of the most surprising facts that you learned about this history?

TARA ROBERTS: There were about 12,000 ships that participated in the slave trade.

What I realized is that when I was growing up, I couldn't tell you the name of a single one of them, but yet I could tell you the name of the Mayflower, I could tell you all about the Titanic, but I couldn't tell you the name of a single one of these 12,000 ships.

That didn't feel right to me.

The other stats on my heart is the fact that approximately 1.8 million Africans died in the crossing from Africa to the Americas.

I thought to myself, who's mourning that loss?

It's an enormous amount of loss.

Who even knows that?

I didn't know that many people had died in the crossing.

It helps me see that there's a whole chapter of history that we just don't know.

ALI ROGIN: We learn in the book that fewer than 1 percent of archaeologists are black.

That seems to have informed what parts of history get covered.

Was that something that you learned more about on this journey?

TARA ROBERTS: Yes, absolutely.

As I talked to and met more and more archaeologists, I actually asked them the same question.

I was like, what difference does it make if there are more black archaeologists or not?

And what they said was really interesting.

They said, we ask different questions.

We see different things.

We're interested in different parts of the story.

So to really get a complex full story, you need all of these pieces.

The other parts are just as important, but.

But this part has really been missing.

So I think it's just really important to have representation from a variety of people that are involved with this.

ALI ROGIN: The book is "Written in the Waters, A Memoir of History, Home and Belonging."

Tara Roberts, thank you so much for being here.

TARA ROBERTS: Thank you for having me, Ali.