Aboard the ship JOIDES Resolution last winter were 30 scientists, 85 crew members, 460 pounds of coffee, hundreds of pieces of million-year-old rock, and one very big drill.

The mission: catalog the history written into the layers of rock below the Mediterranean Sea and the Atlantic Ocean to understand Earth’s climate history.

That goal is what brought UChicago geologist Clara Blättler to the JOIDES Resolution (JOIDES stands for the Joint Oceanographic Institutions for Deep Earth Sampling) in early 2024, along with several dozen other researchers from around the world. With the drill, the scientists can retrieve long, thin cores of the mud and rock below the ocean. Each of the slim cylinders—which measure just over two inches in diameter and about 32 feet in length—is a cross-section of the seafloor, with layers deposited bit by bit over the past several million years.

“So much of our understanding of the world comes from these cores of rock from the seafloor,” says Blättler, an assistant professor of geophysical sciences. “They even helped prove the theory of plate tectonics itself. They are really invaluable.”

The story of the world is recorded at the bottoms of Earth’s oceans: the fall of mountains as they’re washed down rivers a pebble at a time, the rise of new species as seen through their fossilized remains, the shifts of major ocean currents, and the advance and retreat of glaciers.

There are many questions that these cores can help answer, but for this particular JOIDES Resolution expedition, the focus is on the Messinian Salinity Crisis, an event that is scientifically interesting in its own right—and important for understanding today’s climate system.

Six million years ago, the Mediterranean Sea suddenly became a lake. The strait normally connecting it to the Atlantic Ocean was cut off, maybe by tectonic plates moving, changes in global sea level, or a combination of the two.

What’s indisputable is that things got hairy for everything living in the newly minted lake. Without regular water flow from the ocean, the whole sea slowly began to evaporate—and what was left behind got saltier and saltier.

“You can see it in the cores—you come to a layer nearly a kilometer thick that’s just salt,” says Blättler. “There are no fossils in this layer; much like the Dead Sea today, nothing bigger than a microbe could survive in these waters.”

The Messinian Salinity Crisis killed a lot of fish locally, but scientists think it likely affected the climate worldwide as well; normally, the water that flows out of the Mediterranean contributes to global ocean circulation.

Eventually, after about half a mil-lion years, the strait opened up again—possibly in one huge flood—and normal marine life returned.

The saga is particularly of interest to scientists today because the entire Mediterranean region is forecast to become more arid as climate change progresses. The sea is already measuring a little saltier than it was two decades ago.

Exactly what this change will mean is unknown, but looking to the past may provide some clues, Blättler says. “By understanding this really extreme event, we can learn more about how these processes influence climate as a whole.”

Gaining that understanding requires arduous work. For virtually the entire two months the JOIDES Resolution is at sea, the drill is working around the clock. Everyone pulls a 12-hour shift, sharing a tiny cabin with someone on the opposite shift.

When the cores come off the drill, technicians cut them into five-foot sections that are carried to the ship’s lab. There, the scientists scan and measure the properties of each core segment, split it in half to describe everything they see, and check for fossils to get a sense of its age. (The cores will be more precisely dated later.) When the drill is working at its fastest, they have just 45 minutes to get the cores recorded and stored before the next one comes up.

“It’s pretty intense,” says Blättler, who worked the midnight-to-noon shift. “But it’s worth it because this is the only way we have to answer a lot of these scientific questions.”

Now, months later, Blättler and her colleagues can begin analyzing the cores in earnest at their home institutions. The segments will be dated and the overlapping layers matched up to create a comprehensive timeline going back millions of years.

Some of the material will be pulverized and analyzed; for example, Blättler’s lab will investigate the composition of the carbonate minerals in the cores to see how the makeup of the Mediterranean Sea and the Atlantic Ocean changed over time.

And just as NASA has saved some of the precious moon rock samples from the Apollo missions for later studies, half of the cores will be set aside for future analysis, in hopes that new technologies may offer new insights.

While the future of the cores is secure, the fate of the JOIDES Resolution is not: the National Science Foundation announced last year that it was not renewing its agreement with Texas A&M University to operate and maintain the vessel. In August the ship returned from what may have been its final voyage.

Still, Blättler has confidence that her long days at sea, gathering core after core, were well worth the effort: “People will be learning from these for generations.”