An ancient ‘supervolcano’ has been found hiding beneath eastern England. But experts say there’s not enough evidence to call it that.

Scientists have identified an ancient volcano on England’s east coast that is so eroded, it took researchers three decades to piece together its enormous size and explosive past. What remains of the caldera system is located beneath the English counties of Norfolk and Lincolnshire.

However, while the volcano undoubtedly produced at least one huge-scale eruption, some experts don’t think there is enough evidence for the structure to be called a supervolcano.

“It obviously must have been a very large eruption, but it would be very difficult to calculate the volume of the eruption given the poor preservation of the deposits,” said Michael Poland, a research geophysicist with the Cascades Volcano Observatory and the scientist-in-charge of the Yellowstone Volcano Observatory. “This is part of the reason I really hate the term ‘supervolcano,'” Poland, who was not involved in the discovery, told Live Science in an email.

In a recent analysis, published April 29 in the Geological Society of America Bulletin, scientists examined zircon crystals that were taken 80 years ago from boreholes 40 miles (65 kilometers) apart in Norfolk and Lincolnshire. Zircon crystals are smaller than the diameter of a human hair and form within molten magma underground before a volcano erupts. They contain uranium that decays into lead isotopes at a known rate, enabling researchers to precisely date eruptions.

The results showed an age match between the zircon crystals from Norfolk and Lincolnshire and counterparts taken from the Kinnekulle tephra, a thick layer of volcanic ash covering Scandinavia and the Baltic region. This suggests the ash plume from the volcano in what is now eastern England blew across the Tornquist Sea, a body of water that existed during the Ordovician period (488.3 million to 443.7 million years ago), and settled on what is now Scandinavia.

Researchers already knew that the Kinnekulle tephra was formed during a massive volcanic eruption. But the recent analysis shows for the first time how far the ash traveled and, therefore, how big the eruption really was.

The eruption would have also produced huge pyroclastic flows, or avalanches of rocky debris, ash and hot gas that race down the sides of some erupting volcanoes, Peter Rowley, a volcanologist and senior lecturer at the University of Bristol in the U.K. who was not involved in the analysis. “The last eruption of this kind of scale was the Oranui event at Taupō in New Zealand, over 26,000 years ago,” he told Live Science in an email.

However, while the explosion would have been impressive and equivalent to the force of thousands of atomic bombs, experts said the definition of a supervolcano is vague and there are better ways to describe what happened.

“The scientific definition of a supervolcano varies a little; it’s a term we tend not to use as volcanologists much,” Rowley said. “In this case we would describe it as an ultraplinian eruption.”

Ultraplinian eruptions are the most violent type of volcanic explosions, ejecting more than 240 cubic miles (1,000 cubic kilometers) of material. It’s difficult to say exactly what the eruption that formed the Kinnekulle tephra would have looked like, but it likely lasted days to weeks and formed ash clouds taller than 25 miles (40 km) that reached into the stratosphere.

Geologists tend to reject the term supervolcano because it is imprecise, instead preferring to describe eruptions by the volume of material they eject.

Stephen Self, a physical volcanologist at the University of California, Berkeley, agreed that the term supervolcano can be too vague. “Supervolcano has become a kind-of buzz-word,” he told Live Science in an email. “It’s the deposits of the eruptions that are recognized, not the volcano.”

Very little of the material ejected in England’s ultraplinian eruption remains today, because most of it was eroded or buried. “The geological record is far from complete so it can be very difficult to say much about these eruption volumes with very old volcanoes,” Rowley said.

Similarly, the Kinnekulle tephra has been degraded over time, Poland said.

The researchers also found evidence for an earlier eruption of the volcano that occurred 453.7 million years ago. However, there is no trace of these eruptions at the surface today; the boreholes are located in some of England’s flattest landscapes, study first author Tim Pharaoh, a geophysicist with the British Geological Survey, said in a statement in August.

It’s not surprising to find traces of big, ancient volcanoes in England. The recently identified volcano is part of a wider volcanic system that was active 450 million years ago and includes Eryri National Park in Wales and the Lake District in northwestern England, Rowley said.

“It’s best not to imagine whole preserved volcanoes, but rather that we have deposits from their eruptions, or rocks which formed in the magma reservoirs beneath the original volcano,” Rowley said. “These are often deformed and altered by tectonic events meaning that it takes some careful geological investigation to interpret the true picture.”