Moho-A-Go-Go: Journey to the Far Edge of the Center of the Earth

In May 2023, the first samples of the Earth’s mantle, the silicate rock layer between the crust and outer core of our planet, were pulled up by the research vessel JOIDES Resolution. The rocks were taken from where they had welled up in the mid-Atlantic’s Atlantis Massif’s “tectonic window,” putting them unusually close to the ocean floor. It was a milestone—actually a kilometer of serpentinite mantle rock—reached six decades after the first attempt to tap the mantle.

Jules Verne might have had people journeying to the center of the earth in the 1860s, but outside of science fiction, humans have barely explored the lithosphere, the rocky outer layer of the planet, much less its deeper regions. The deepest humans have reached is 7.6 miles at the Kola Superdeep Borehole near the Norway border, a project that ended with the collapse of the Soviet Union. The average distance to the center of the Earth, by the way, is 3,959 miles.

In the late 1950s, the American Miscellaneous Society (AMSOC)—yes, that’s what they called themselves—came up with an idea to reach the Moho, the presumed border between the crust and the mantle below it.

“Moho” is short for the Mohorovičić discontinuity, named for Croatian seismologist Andrija Mohorovičić. In 1909, Mohorovičić noted a distinct change in the movement of seismic waves as they passed through the lithosphere. This was because lighter density crust essentially floats atop higher density mantle, and the densities of these different types of rock determine the way seismic waves move. This border or discontinuity is an average of 3–6 miles beneath the ocean floor and 10–60 miles beneath the continents.

So, to reach it, you drill down, preferably in the ocean. Ocean drilling is, it should be stressed, more complicated than drilling on land. In what perhaps inevitably became known as Project Mohole, the quirky collection of scientists who made up AMSOC would drill down through the ocean floor, where the crust is thinnest, in the waters off Baja California.

The National Science Foundation founded Project Mohole. John Steinbeck, a year away from his Nobel Prize in literature, went along for the ride. He wrote up the story for Life magazine in 1961, “High Drama of Bold Thrust through Ocean Floor.” For a brief moment, the interest in the space race covered both inner and outer space. The earth sciences, still reverberating with the achievements of the International Geophysical Year of 1957–1958, were very much a component of planetary exploration. AMSOC was boldly drilling where no drill had gone before.

But the Moho-a-go-go was not to be. The test drilling got through a lot of sediment and then 13 meters of basalt, but the mantle remained inviolate. “Recovering subseafloor basalt was a major scientific achievement at the time,” earning the team a congratulatory telegram from President John F. Kennedy. But funding was cut off by 1966. The projected costs of a purpose-built drilling rig and then actually punching all the way down were, to flip a phrase, astronomical.

Steinbeck wrote that we knew less about the ocean floor than we knew about the Moon. This line still gets repeated, often in the more generic form of “the ocean” rather than “the ocean floor,” as if this was still the case. In her recent tour de force of how the ocean works, The Blue Machine, physicist and oceanographer Helen Czerski argues that that perspective couldn’t be more wrong. We actually know a lot more about the ocean, and the ocean floor, than we know about the Moon, not least because there’s a lot more to know about the ocean than there is about the Moon, which is, after all, just a lifeless rock.

So, while humans have walked on that lifeless rock, we still haven’t reached the Moho. And as for the center of the Earth? Fuhgetaboutit, it’ll never be open to tourism. This hypothetical trip down, though, is pretty interesting, and really, really hot.

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