In the past, even with an icebreaker and during peak melt season, getting to the North Pole wasn’t a sure bet. It took favorable winds to crack the frozen ocean surface, and ships had to fight through ice that had grown many meters thick over several winters. In the summer of 2025, though, Jochen Knies from the Arctic University of Norway, Tromsø, and his team met little resistance on their way to 90 degrees North with the research vessel Kronprins Haakon. The geologist “didn’t hear the usual grinding of ice” against the hull that he remembered from 1996, when he first reached the pole by ship. Instead, thin floes and large stretches of open water made for an easy, quiet passage. To him, it was “a reminder of how quickly the Arctic is changing.” Since the late 1970s, when satellite observations of the polar seas began, summer ice cover of the Arctic Ocean has declined by more than 40%. In less than half a century, a frozen area the size of the Mediterranean Sea has turned into blue open water with the rapid warming of the high northern latitudes. If this trend continues, there could soon be summers at the North Pole with no sea ice whatsoever. The last time this happened may have been some 120,000 years ago. But no one knows for certain. That’s why Knies and his colleagues, a team of researchers from Norway and Germany, set out from Svalbard to the central Arctic last August. The aim of their five-week mission was to determine whether this region had been ice-free in recent Earth history—and if so, when. As part of a €12.5 million project financed by the European Union, they also came to answer some questions about the future of the Arctic and beyond: How does the loss of sea ice affect the marine ecosystem? What are the consequences for ocean circulation and global climate? In search of clues, the expedition collected sediment cores up to 22 meters in length at different locations across the Arctic seafloor. Marine sediments are valuable climate archives that give scientists a window into bygone eras. Like diligent record keepers, they can log past water temperatures, sea-ice coverage, and the strength of ocean currents. These data are encrypted in the chemical and physical properties of the plankton remains and weathered rock deposited on the seabed.  The ship’s crew and researchers recover the sediment corer, a 25-meter-long steel pipe that is driven into the seafloor using a top weight of more than three metric tons.TIM KALVELAGE Together, the scientists pull out long plastic pipes filled with precious deep-sea mud.TIM KALVELAGE The pipes are cut into shorter pieces and split in half before being processed in the ship’s laboratories. Each of these one-meter sections covers several tens of thousands of years of Earth’s history.TIM KALVELAGE While sediment cores several meters long had been recovered on earlier expeditions in the central Arctic, there is no scientific consensus on how old the deposits actually are or whether sea ice ever completely disappeared in summer.  To decode the Arctic’s climate archive, Knies brought a team of experts from various disciplines onboard the Kronprins Haakon to dig deeper and obtain fresh samples they could subject to the latest analytical techniques.    Samples await paleomagnetic dating. Like tiny compass needles, iron-rich particles align with Earth’s shifting magnetic field as they settle on the seabed. By measuring their orientation, researchers can estimate the age of the different sediment layers.TIM KALVELAGE Under the microscope, PhD student Paulina Romel picks shells of unicellular foraminifera from a sample. The chemical composition of these microfossils can give clues about the age of the sediment and the surface water temperature when the organisms were still alive. “These are really cool creatures!” says Romel.TIM KALVELAGE Agathe Ollive, a geochemist from the Alfred Wegener Institute in Germany, takes water samples from a CTD rosette, an instrument package that measures conductivity (salinity) and temperature at various depths. She uses certain elements to trace the inflow of fresh water and seawater from rivers and adjacent ocean basins into the Arctic. “I didn’t expect there to be so little ice up here,” Ollive says. She is worried about how the Arctic will look 20 years from now.TIM KALVELAGE Some of this work was done while the researchers were still at sea. Now, at their home laboratories, they are finalizing their analysis of the seafloor samples. One important task is dating the sediments, which may be up to 2 million years old. The team uses a combination of methods to do this, including measuring magnetization, the decay of radioactive elements, and the exposure of mineral grains to sunlight before sinking to the depths. Once they can place them on a timeline, the materials in the cores will help researchers paint a picture of what the Arctic Ocean looked like in times that were warmer than today. For example, the presence or absence of the molecule IP25, which is produced exclusively by ice algae, could tell them how far the sea ice receded at a given time.  Toward the end of the expedition, the Kronprins Haakon passes this iceberg near the northeast coast of Greenland.TIM KALVELAGE At the end of the study, the team hopes to have data that could improve climate projections for a future ice-free “blue Arctic,” helping us understand how it could affect marine life and carbon storage, Atlantic Ocean circulation, or extreme weather events in Europe and North America.  Tim Kalvelage is a freelance science reporter based in Bremen, Germany, who focuses on climate, ocean, and polar research. He has been to the North Pole twice. ...read more read less