We studied a layer of volcanic ash found in a seafloor sediment core collected from the Bismarck Sea. At first, we wanted to find out if this ash had affected ocean life or climate. But instead, our research revealed something else: the ash likely came from a powerful underwater volcanic eruption, similar to the 2022 Hunga Tonga eruption. The ash layer, about 3,000 to 4,000 years old, contains small glassy particles and dark-colored marine shells, which can form from intense lightning in giant eruption clouds. While we expected signs of increased ocean productivity, we didn’t find clear evidence of that. Still, our results suggest the ash came from a nearby underwater volcano about 90 kilometers away—not from more distant ones like Rabaul or Tavui. This study shows that deep-sea eruptions can leave long-lasting clues in ocean sediments. It also highlights how much we still have to learn about how these eruptions affect ocean life.