In her new book, “The Brilliant Abyss,” Helen Scales writes about the largely unseen realm of the deepest parts of the ocean. On this week’s podcast, she talks about the life down there — and how long it took us to realize there was any at all.

“It wasn’t so long ago, maybe 200 years ago, that most people — scientists, the brightest minds we had — assumed that life only went down as far as sunlight reaches, so the first 600 feet or so,” Scales says. “But what’s so fascinating is that life does go all the way to the very, very bottom; down to seven miles, which is the deepest point, just about. And there are ways in which life has found adaptations to all of these crazy, extreme conditions in the deep, and that’s what we’re really doing a lot of the time, as marine biologists working in the deepest, is finding that stuff and asking the question: ‘How are you here?’”

Rebecca Donner visits the podcast to discuss her new book, “All the Frequent Troubles of Our Days,” which recounts the story of Mildred Harnack, Donner’s great-great-aunt, an American woman executed in 1943 for being a member of the German resistance to the Nazis during World War II.

“She most definitely saw herself as a resistance fighter, and she certainly did not see herself as a spy,” Donner says. “She engaged in acts of espionage in order to undermine the Nazi regime, but she never met with a control officer, she never accepted money. She worked in an unofficial capacity.”

Also on this week’s episode, Tina Jordan looks back at Book Review history as it celebrates its 125th anniversary; Elizabeth Harris has news from the publishing world; and Gal Beckerman and John Williams talk about what they’re reading. Pamela Paul is the host.

Here are the books discussed in this week’s “What We’re Reading”:

“Four Thousand Weeks” by Oliver Burkeman

“Ghettoside” by Jill Leovy

“Last Best Hope” by George Packer

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Life at Seven Miles Below the Sea

The Book Review