Hey, it's Empire's Book of the Day. I'm Andrew Limbaugh. The Journey Inward is a journey into darkness.

It is not a journey of conquest. It is really one of submission. This quote is not about finding

inner peace. It isn't someone talking about healing from trauma or understanding generational pain.

It's about scuba diving.

Granted, it's about really deep scuba diving, so deep that I imagine it's spiritual on some level.

The guest on the pot today, Susan Casey, says it while talking about why space exploration

gets all the buzz over the deep sea.

For Earth Day, we've got her talking about her book,

The Underworld Journeys to the Depths of the Ocean. And in this conversation with NPR's A.

Martinez, she talks about what's so alluring down there and why we need to treat the deep ocean

with humility. That's after the break. In the U.S., national security news can feel far away from daily life.

Distant wars, murky conflicts, diplomacy behind closed doors.

On our new show, Sources and Methods.

NPR reporters on the ground bring you stories of real people,

helping you understand why distant events matter here at home.

Listen to sources and methods on the NPR app or wherever you get your podcasts.

It's easy to dream about space because all you got to do is go outside and look up to see some of what's out there.

But the deep ocean, that's not so easy because even if you're in the ocean, you can't just

look down and see what's in the deep waters. Typically, it's defined as the waters below 600 feet.

So as you can imagine, since it goes all the way down to almost 36,000 feet in places, the deep

ocean is the vast, vast, vast majority of the ocean. Susan Casey has been

down there. She's a diver and author of the new book, The Underworld, journeys to the depths

...