The Polarella glacialis is a species of phytoplankton, microscopic algae, that is present in both arctic and antarctic coastal and marine environments. This microscopic algae commonly found in first-year sea ice sinks to the bottom of the sea once the ice melts. A team of scientists analyzed marine sediment samples and found that the presence of DNA from the Polarella glacialis is linked with the concentration of sea ice observed. They were also able to find the same DNA in sediments dating from 12 000 years in northern Baffin Bay, which allows them to know what sea-ice concentrations were at that time! Their new method could provide insights on the sea-ice conditions that both poles faced over the last millennia. The paper describing this work by Harðardóttir et al. is currently under review at Nature Portfolio.