Researchers from Kobe University have identified what may be the largest volcanic eruption ever recorded by humans. The event, known as the Kikai Akahoya underwater eruption, occurred about 7,300 years ago off the western coast of Japan. The discovery was reported in a high profile geoscience study that focuses on long buried volcanic activity and its climatic legacies.
Historical records and geological clues indicate that the Kikai volcanic system has produced several substantial eruptions over the last 14,000 years. The most powerful had a magnitude well beyond anything seen in modern times, releasing far more ash and rock into the ocean depths than the famous Tambora eruption that occurred in 1815 in Indonesia. This new finding reframes our understanding of the scale of eruptions in the prehistoric record and highlights the extraordinary forces unleashed by volcanic systems beneath the sea floor.
To build a clearer picture, scientists mapped the seafloor around the Kikai zone using seismic data and modern remote sensing techniques. They drilled multiple sites with remotely operated vehicles and retrieved sediment cores that offered a direct record of the ancient eruption. The analysis of these underwater sediments revealed a vast, continuous layer spanning thousands of square kilometers and composed largely of volcanic glass and other eruptive debris. The material recovered points to an enormous seismic and explosive event that deposited about 71 cubic kilometers of volcanic fragments into the ocean, a figure that surpasses earlier estimates published in prior studies.
When researchers integrated the underwater findings with estimates of debris that settled on land, the total ejecta from the eruption rose dramatically. The combined data suggest that roughly 457 cubic kilometers of material were expelled in total, some of which would have impacted distant landscapes far from the shoreline. By comparison, this volume is enough to fill North America’s Lake Tahoe twice over, illustrating the vast reach of this ancient explosion and its potential to influence regional landscapes and climate patterns at the time.
The Kikai Akahoya event stands among the most powerful known prehistoric eruptions. For context, the Toba supervolcano eruption in Sumatra around 74,000 years ago ejected tens of thousands of cubic kilometers of magma, a scale that dwarfs most modern eruptions and continues to shape scientific discussions about volcanic forcing on global climate.
Earlier research has also uncovered the presence of giant submarine volcanoes off other coastlines, including regions off the western coast of South America, underscoring a common thread in Earth systems where megaplumes and colossal underwater eruptions leave lasting imprints on the seafloor and in surrounding sedimentary records. This growing body of work helps scientists better assess the frequency, scale, and environmental impact of underwater volcanism across different ocean basins. [Citation attribution: study findings discussed in contemporary geoscience literature, with data and interpretations supported by seismic mapping, drilling results, and sediment analysis]