One of the last things you'd think could be lost is a volcano. Yet, when we look at the record of volcanic activity, even in the past few hundred years, there are signals that a large eruption occurred ... we just don't have any other evidence of the source of that big blast. Slowly but surely, through geochemical sleuthing, some of those mystery eruptions have been matched up with volcanoes. It turns out that many times, that source is more surprising than anticipated.A recent study in the Proceedings of the National Academy of Sciences (PNAS) by William Hutchinson and others may have found the smoking gun for a mysterious event recorded in the planet's ice cores. If we look at the signal of climate-altering eruptions since 1800 captured by ice core records on both poles, many famous eruptions show up: Tambora (1815), Krakatau (1883), Pinatubo (1991) and many others. However, certain peaks didn't have obvious matches in the known geological or historical records."Missing" VolcanoesRinjani caldera in Indonesia. Credit: Wikimedia Commons / DhmlombokOne of the most famous "missing eruptions" was one that occurred around 1257-58 CE. It was one of the largest sulfur-deposits events in the last few thousand years but up until recently, it had no known source. Work by Franck Lavigne and others in 2019 determined using geochemical comparisons of ash from a hitherto unmapped eruption of Samalas in the Rinjani caldera of Indonesia as the culprit. So, this "missing" eruption was hiding in plain sight.However, it sometimes isn't that simple. Rinjani is in Indonesia and currently has over 3,000,000 people living near it. Back when it erupted in the 1200s, the population wasn't nearly as high, but the regional capital at the time was located near the volcano. Records show that the capital was destroyed by ash and the locals could definitely point out the copious ash deposits all around the area. It just took the geochemical analyses of the ash near the volcano and in the ice core to correlate these two events.Now, suppose a volcano erupted and no one was watching? Hutchinson and others' choice for the most probable source of sulfur spike in the ice core record in 1831 is potentially a volcano that no one saw erupt. Using similar geochemical methods to compare ash fragments, they have pointed to the remote Zavaritskii caldera in the Kuril Islands south of Kamchatka in the western Pacific. The Middle of NowhereA Sentinel-2 image taken March 3, 2024 of Simushir Island in the Kuril Islands between Japan and Russia. The four volcanoes on the island are labeled. Credit: ESA, annotated by Erik KlemettiIf you've never heard of the volcano, you're not alone. Looking at the Global Volcanic Program's database, Zavaritskii has 118 people currently living with 100 kilometers of the volcano. That is no typo: 118 people. Back in the mid-1800s that number was likely closer to nobody ... and unlike today, where ships and jets pass near the region everyday, there was likely almost no ships in the area at the time as well. You have the perfect situation for a large eruption that no one saw happen.It isn't surprising that in such a remote location, there hasn't been much research done on the eruption history of Zavaritskii. There were a few small eruptions in the 1900s, but the history prior to that was pretty vague. Before this new research, the caldera and dome forming eruption was thought to have occurred "in the Holocene" ... which means the last 10,000 years or so. Not exactly the precision you want to match with a big sulfur spike. However, Hutchinson and others were able to collect some new radiocarbon ages for material caught up in this last major eruption and found it was less than 300 years ago.Up until now, the best guess volcanologists had for the source of the sulfur in the ice cores around 1831 was a very small eruption of Ferdinandea (part of the Campi Flegrei Del Mar di Sicilia) off the coast of Sicily (not to be confused with Fernandina in the Galpagos Islands). Even though it was a small event, the magma incorporated sediments rich in anhydrite, a mineral formed by the evaporation of water that is rich in sulfur. So, without a better source, that was the guess.Isotopic FingerprintsCompositional data of Zavaritskii Caldera Ash showing the composition match of ice core ash and material from the Zavaritskii caldera. Credit: Hutchinson and others, PNAS.However, Hutchinson and others were able to use the sulfur isotopic composition of the ice core and ash from Zavaritskii to show that Ferdinandea couldn't be the source. The ratio of sulfur isotopes in anhydrite, the supposed source of sulfur in the Ferndinandea eruption, is very different than that in normal magma erupting in a place like the Kuril Islands. The mismatch of sulfur isotopes from that of anhydrite and the close match between the ice core sulfur isotopes and Zavaritskii ash put the nail in the coffin of the 1831 sulfur spike source.It turns out that this eruption that doesn't appear to show up in any known historical record was likely comparable to the 1991 eruption of Pinatubo. There were plenty of associated climate and atmospheric impacts recorded at the time, including dimmed sunlight, blue hazes and changed weather patterns. The amount of sulfur thrown into the atmosphere played a role in crop failures that led to famines in the mid-to-late 1830s in Japan and India. This isn't the last "missing" volcanic eruption. There are a pair of sulfur spikes in ice core records in the early 1800s that have yet to be conclusively linked to a specific volcano. When you look back at some of the most significant explosive eruptions over the past few centuries, many have come from volcanoes that weren't recognized to have the potential: Pinatubo, El Chichn, Chaitn, Huaynaputina and more. This is why any list of the "next" volcanoes to cause such an eruption are dubious at best. Many of the volcanoes that caused climate havoc were ones that hadn't had a big eruption in millennia, so they weren't closely studied. This goes to show how vital basic mapping, geochemical analyses and dating of volcanoes is to understanding potential volcanic hazards. You never know when that seemingly obscure and quiet volcano could be the next big one.