Susan Casey has traveled about 17,000 feet deep into the ocean – and in her book The Underworld: Journeys to the Depths of the Ocean, the diver and author speaks with oceanographers, marine biologists and geologists to explain some the of the wonders that exist way beyond what we can see in the water. For our Earth Day episode, Casey speaks with NPR's A Martinez about the millions of shipwrecks that are still preserved underwater, the creatures that call the deep ocean home and the humility it takes to learn about the sea.

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