A groundbreaking trip to drill into rocks at the bottom of the Atlantic Ocean has given scientists their best glimpse yet into what Earth might look like beneath its crust.

Researchers have collected a nearly unbroken 1,268-meter-long sample of green, marble-like rock from a region where Earth's mantle - the thick, inner layer that makes up more than 80% of the planet - has penetrated the ocean floor. The rehearsals, which take place on August 8thSciencehave been described provide unprecedented insight into the processes that lead to crust formation.

"We had this story in our head" about what such rocks were supposed to look like, but it's completely different when "you see it there on a table," says Natsue Abe, a petrologist at the Japan Agency for Marine-Earth Science and Technology in Yokohama.

The expedition's achievements are a "fantastic milestone," says Rosalind Coggon, a marine geologist at the University of Southampton, UK. “Ocean drilling provides the only access to samples from deep within the Earth, which are critical to understanding the formation and evolution of our planet.”

Scientists in hard hats and face masks examine core samles in a lab on board JOIDES Resolution research vessel

Earth's oceanic mantle—the type of crust found primarily beneath Earth's seas rather than beneath its continents—is composed primarily of dense, volcanic rock called basalt. It is much thinner and younger than the continental crust because the rocks are continually recycled through the movements of tectonic plates.

Basalt forms when magma pushes up through submarine fissures along mid-ocean ridges. The magma itself comes from a process called partial melting in the mantle - which is largely made up of translucent, green, magnesium-rich minerals. As the material rises in the mantle, the pressure above it falls, causing some of these minerals to melt and form microscopic films of magma between rock crystals.

Normally only magma erupts onto the ocean floor. But in some places, mantle rock also makes it to the surface, where it comes into contact with seawater in a reaction called serpentinization. This changes the structure of the rock - giving it a marble-like appearance - and releases various substances, including hydrogen.

Easy to drill

Visited in May 2023JOIDES Resolutiona place where this happened: an underwater mountain called the Atlantis Massif, located west of the mid-ocean ridge of the Atlantic. The 143 meter long ship is equipped with a 62 meter high crane for underwater drilling.

The researchers on board decided to Lost City to drill, a location on the south side of the massif. The region is characterized by hydrothermal vents in which microbial extremophiles use the escaping hydrogen.

“We only planned to drill 200 meters because that was the deepest place humans had ever managed to drill into mantle rock,” says Johan Lissenberg, a petrologist at Cardiff University, UK. But the drilling was surprisingly easy and three times faster than usual, and it returned long, unbroken cylinders of rock called cores. “So we just decided to keep going,” Lissenberg says. The team only stopped when the expedition ended according to its schedule.

The researchers have now published their first results. "What we're reporting is literally what you can do on the ship. A team of 30 scientists poring over the cores day and night for two months, documenting, inch by inch, how they come up."

Deep-sea drilling: Diagram showing how researchers on a ship drilled into rock that originated in the Earth's mantle.

When scientists examined the rock's structure in detail, they noticed "slanted features," a characteristic signature of the prevailing theory of how magma separates from the mantle to become part of the crust, Lissenberg says. The mantle rock was also interspersed with other types of rock in the cores, suggesting that the mantle-crust boundary is not as sharp as seismographic data typically suggests, says Jessica Warren, a geochemist at the University of Delaware in Newark. These results together “are crucial to how we understand the formation of tectonic plates in the oceans,” she says.

Uncertain future

The trip caps a worthy four-decade career for theJOIDES Resolution, which the US National Science Foundation (NSF) had rented from a private company. But the NSF has announced that it can no longer afford the $72 million per year it costs to operate the ship after meeting its IODP obligations and that the program will be terminated. That leaves some scientists, particularly those early in their careers, uncertain about the future of the field, says Aled Evans, a marine geologist at the University of Southampton.

A remaining “grand challenge” for geoscientists is drilling through the basaltic layer and across the crust-mantle boundary – called the Mohorovičić discontinuity, or “Moho.” This would allow them to access virgin mantle rocks that have not reacted with seawater. “We haven’t drilled into the real mantle yet,” Abe says. The surprisingly smooth drilling at Lost City bodes well for these future attempts by Japan's research vesselChikyucould be carried out, she adds. “Mantle rocks are the most common part of our entire planet,” says Evans. “Sampling them would be fundamental to understanding what our planet is made of.”