The H.M.S. Challenger spent just over three years in the late 1800s circumnavigating the globe and studying ocean life. Today, thousands of the specimens collected during the Challenger expedition reside at London’s Natural History Museum.

In a new study published in Scientific Reports, researchers at the museum compared plankton collected on the Challenger expedition to modern specimens. They found that on average, today’s plankton have thinner shells than those from 150 years ago—likely as a result of climate change.

The Challenger was a small warship before it was a research vessel. Fifteen of its 17 guns were removed before the mission to make more space on the ship, which was then decked out with rooms for photography, dissections, laboratories and a small library. The Challenger set off in December 1872 with thousands of bottles and boxes for holding specimens and 181 miles of rope to measure the ocean’s depth.

Throughout the journey, the crew would dredge the bottom of the ocean with a weighted net. The net brought up fish, mud and at least one shark. The six scientists aboard the ship documented everything they found, big and small.

“The mud! Ye gods, imagine a cart full of whitish mud, filled with minutest shells, poured all wet and sticky and slimy on to some clean planks,” Sublieutenant Lord George Campbell wrote of the dredging, per Discover magazine’s Kate Golembiewski. “In this the naturalists paddle and wade about, putting spadefuls in successively finer and finer sieves, till nothing remains but the minute shells.”

The smallest among these, at less than one millimeter wide, would have been the two species of shelled plankton that micropaleontologist Lyndsey Fox, a postdoctoral researcher at the Natural History Museum, found in the museum's collections. Using the Challenger expedition’s thorough field notes, Fox and her colleagues were able to pinpoint where the historical samples were collected. Then they found samples of the same species at the same place during France's Tara ocean expedition in 2011.

They found that plankton shells today are much thinner than they used to be. In one species, the modern shells were 76 percent thinner.

“I was a little bit shocked to see how dramatic the results were for some species,” Fox tells Science magazine’s Erin Malsbury. In some cases, she says, the shells were so thin that the team was unable to capture clear images of them.

The tiny sea creatures’ shells are made of calcium carbonate, a dusty white mineral also used as chalk. Previous research has shown that as carbon dioxide builds up in the atmosphere and the oceans become warmer, some of that carbon dioxide gas mixes into the water. There, it becomes carbonic acid and makes it more difficult for shelled marine life, like corals, sea urchins, clams and plankton to build their shells.

While lab experiments have shown before that this ocean acidification would damage marine life, this is the first study to illustrate the real-world effects over more than a century.

“This is a really neat demonstration,” says University of Bremen paleo-oceanographer Lukas Jonkers, who wasn’t involved in the study, to Science, adding that there’s potential to study more specimens from a wider variety of locations. “They’re sitting on a unique treasure there at the Natural History Museum.”

The Challenger’s thorough notes have been used before to study the effects of climate change on the ocean. In a study published in Science in 2019, researchers from the Woods Hole Oceanographic Institute described how temperature readings taken by the Challenger and today show that while the ocean’s surface is warming, the deep ocean is still recovering from the “Little Ice Age.”

The latest findings are a bad sign for ecosystems that rely on plankton at the bottom of the food chain.

“If the foraminifera [plankton] are struggling, then that is going to cause a knock on effect to the larger creatures that consume the plankton and the predators that subsequently feed on them,” co-author Stephen Stukins said in a statement. “This will inevitably lead to species extinctions.”