Scientists think they have found the volcano responsible for a huge eruption that occurred in the 13th Century.
The mystery event in 1257 was so large its chemical signature is recorded in the ice of both the Arctic and the Antarctic.
European medieval texts talk of a sudden cooling of the climate, and of failed harvests.
In the PNAS journal, an international team points the finger at the Samalas Volcano on Lombok Island, Indonesia.
Little remains of the original mountain structure - just a huge crater lake.
The team has tied sulphur and dust traces in the polar ice to a swathe of data gathered in the Lombok region itself, including radiocarbon dates, the type and spread of ejected rock and ash, tree-rings, and even local chronicles that recall the fall of the Lombok Kingdom sometime in the 13th Century.
"The evidence is very strong and compelling," Prof Clive Oppenheimer, from Cambridge University, UK, told the BBC's Science In Action programme.
Co-worker Prof Franck Lavigne, from the Pantheon-Sorbonne University, France, added: "We conducted something similar to a criminal investigation.
"We didn't know the culprit at first, but we had the time of the murder and the fingerprints in the form of the geochemistry in the ice cores, and that allowed us to track down the volcano responsible."
The 1257 eruption has been variously linked with volcanoes in Mexico, Ecuador and New Zealand.
But these candidates fail on their dating or geochemistry, the researchers say. Only Samalas can "tick all the boxes".
The team's studies on Lombok indicate that as much 40 cubic kilometres (10 cubic miles) of rock and ash could have been hurled from the volcano, and that the finest material in the eruption plume would likely have climbed 40km (25 miles) or more into the sky.
It would have had to be this big in order for material to be carried across the entire globe in the quantities seen in the Greenland and Antarctic ice layers.
The impact on the climate would have been significant.
Medieval texts describe atrocious weather the following summer in 1258. It was cold, and the rain was unrelenting, leading to flooding.
Archaeologists recently put a date of 1258 on the skeletons of thousands of people who were buried in mass graves in London.
"We cannot say for sure these two events are linked but the populations would definitely have been stressed," Prof Lavigne told BBC News.
In comparison with recent catastrophic blasts, Samalas was at least as big as Krakatoa (1883) and Tambora (1815), the researchers believe.
The ice cores do hold clues to yet another colossal event in about 1809, but, like Samalas before it, finding the source volcano has been difficult.
Prof Oppenheimer said: "It's outstanding that we haven't come across evidence for it. Where in the world could you bury such bad news?"
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos