The new expedition, by an ocean drilling vessel called the JOIDES Resolution, did not technically drill into the mantle, and the hole isn’t the deepest ever drilled beneath the ocean floor. Instead, researchers cruised to a special “tectonic window” in the North Atlantic where drills don’t have to tunnel as far to strike pay dirt. Here, the rocks of the mantle have been pushed close to the surface as the ocean floor slowly pulls apart at the nearby Mid-Atlantic Ridge.

On May 1, they began drilling the hole, known as U1601C. Andrew McCaig, the expedition’s co-chief scientist, expected to make a shallow “pinprick” because the record for drilling in mantle rock, set in the 1990s, was a mere tenth of a mile. The researchers hoped to recover enough samples to help elucidate how chemical reactions between mantle rocks and water could have given rise to life on our planet. But ocean drilling can be an uncertain enterprise — drills get stuck, or the long cores of rock being recovered may be only partial samples.

This time, though, the drill yielded tube after tube of dark rock, many of them surprisingly complete.

“It just kept going deeper, deeper and deeper. Then everyone in the science party said, ‘Hey, this is what we wanted all along. Since 1960, we wanted to get a hole this deep in mantle rock,’” McCaig said, speaking from the JOIDES Resolution minutes before another long section of dark rock was pulled on board. When the team stopped drilling on June 2, the team had taken rock samples from as deep as 4,157 feet below the seafloor.

“We’ve achieved an ambition that’s been feeding the science community for many decades,” McCaig said.

Scientists on land have been eagerly keeping tabs on the expedition, anticipating a jackpot of data that will open a new window into the deep Earth and fuel years of research.

“We are just to the moon with excitement about what they’ve got — an amazing section of rocks,” said Andrew Fisher, a hydrogeologist at the University of California at Santa Cruz, who advises a graduate student who is aboard the ship and has been monitoring their progress remotely.

Making it to the Moho

In 1909, a Croatian seismologist named Andrija Mohorovičić discovered a boundary within Earth.

Mohorovičić monitored how seismic waves generated by an earthquake traveled through the ground, similar to using X-rays to probe inside the human body. Closer to the surface, seismic waves traveled at one speed, but past a certain zone all around the globe, they traveled faster, suggesting the waves were moving through two distinct layers of rock.

This discontinuity, called the Moho, is now recognized as the line between Earth’s crust and its mantle. Its depth varies, but the mantle generally begins about five miles beneath the ocean floor and roughly 20 miles beneath the continents.

“Think of the crust in the way that you have a beautifully iced cake, but what you want is the cake, not the icing,” said Jessica Warren, a professor of Earth sciences at the University of Delaware who has also been monitoring the project’s progress remotely. “If we want to understand the Earth as a whole, there’s a huge, huge amount of rock below that.”

Earth’s inner core seems to be slowing its spin

The mantle isn’t a complete unknown. Occasionally, volcanic eruptions spew out bits of it — chunks of greenish peridotite, the type of rock that dominates the upper mantle, embedded in basalt rock. But these samples, called mantle xenoliths, have their limits, because they are often chewed up and weathered from their trip to the surface. There are also ophiolites, sheets of oceanic crust tinged with some of the upper mantle that were uplifted and plastered onto the land. But they too have been altered by the trip.

What scientists have long craved was a drilled sample of mantle rock. Project Mohole, a famous ocean expedition, set out to drill through the thinner crust on the ocean floor to reach the mantle in 1961 but failed.

Portions of the ocean floor where the mantle is closer to the surface seemed like an opportunity to take a sample without the technical difficulties of drilling through miles of crust. That’s where the scientists aboard the JOIDES Resolution set their sights for one of the vessel’s last missions before its scheduled retirement in fiscal year 2024.

The team departed Ponta Delgada in Portugal’s Azores Islands in April and headed to the Atlantis Massif, an underwater mountain about the size of Mount Rainier. Its primary mission wasn’t to drill the deepest hole yet in mantle rock, but to sample those rocks for clues about how, in the absence of life on infant Earth, small organic molecules might have formed as rocks reacted with water.

“This could be a way that you go from just having basically water and rock,” said Susan Lang, the co-chief scientist of the expedition and a scientist at the Woods Hole Oceanographic Institution. “That produces hydrogen, [and] that hydrogen is a really big fuel to things like the formation of smaller organic molecules, and that can then combine with other organic molecules and lead to early life.”

Going deeper and getting fresher

The rock cores extracted from hole U1601C are dominated by peridotite, the most common type of rock found in the upper mantle. The samples have been altered by their exposure to seawater, and scientists are already beginning to debate how to interpret the findings.

Most of the mantle is buried beneath the crust, not exposed to the ocean the way it is at this site. That raises the fundamental question: How closely do the latest samples mimic the rest of the mantle? Do the rocks truly represent mantle, or are they lower crust?

And for that matter, is the boundary between mantle and crust a sharp boundary, or more of a gradual transition? The samples aren’t pure peridotite, and that could be a key piece of evidence.

“It’s a bit of a hash, but that’s maybe what the lower crust is,” Fisher said, listing off various types of rock that have been reported in daily science logs. “This is really unusual — more than a kilometer of highly altered, lower crustal and/or upper mantle rock. I’d say it’s a mix.”

The scientists have been so busy processing the enormous volume of rock they’ve recovered that they’ve had little opportunity to study the samples in detail, or even reflect on the magnitude of the achievement. The drill bits need to be switched out every 50 hours. The team aboard works in 12-hour shifts, not wasting a minute of time.

On a recent morning, Lang became distracted and excused herself from an interview when she saw seawater spray through a window.

“I saw this seawater stage, which is always a very dramatic point where they detach this one thing and a bunch of seawater sprays everywhere,” Lang said. “Usually, that’s my warning that a core is coming on deck in about the next five minutes.”

What excites all of them is the hope that the deepest samples will yield even “fresher” rock, less altered by other processes and closer to what the mantle is really made of.

“The deeper we get in there, the closer we’re getting to what we those rocks look like, closer to what the mantle looks like,” Warren said.