Methana volcano looked dead for more than 100,000 years, yet magma kept building below ground. By dating tiny crystals, scientists found a hidden system still active, raising new doubts about how safely “extinct” volcanoes are judged around the world.

For more than 100,000 years, the Methana volcano in Greece appeared quiet and lifeless. No lava spilled down its slopes. No ash clouds darkened the sky. To anyone watching from the surface, the volcano seemed extinct.

But deep underground, the story was very different.

An international research team led by ETH Zurich has revealed that Methana was not dead at all. Instead, enormous amounts of magma continued building beneath the volcano during its long silent period. The findings challenge one of the most common assumptions in volcanology: that a volcano quiet for tens of thousands of years is no longer dangerous.

The study paints a striking picture of a volcano slowly “breathing” underground while remaining calm at the surface. Researchers say this hidden growth may happen at many volcanoes worldwide, especially in regions shaped by subduction zones, where one tectonic plate sinks beneath another.

“We can think of zircon crystals as tiny flight recorders. By dating more than 1,250 of them across 700,000 years of volcanic history, we’ve reconstructed the volcano’s inner life with a precision and statistical power that simply wasn’t possible a decade ago,” said Olivier Bachmann, professor of Volcanology and Magmatic Petrology at ETH Zurich. “What we learned is that volcanoes can ‘breathe’ underground for millennia without ever breaking the surface.”

Methana sits less than 60 kilometers from Athens and forms part of the South Aegean Volcanic Arc. The volcano has produced at least 31 eruptions over the last 700,000 years. Most were gentle lava-producing events, though several were explosive.

To understand the volcano’s past, scientists focused on zircon crystals trapped inside volcanic rocks. Zircon forms as magma cools underground. Each crystal preserves chemical and timing clues from the moment it grew.

Using advanced dating methods, researchers analyzed more than 1,250 zircon crystals. They also examined isotopes in plagioclase crystals and bulk rock chemistry. Together, the data created a detailed timeline of Methana’s underground activity.

The results showed that magma production beneath the volcano was almost continuous for hundreds of thousands of years. Even before Methana’s first known eruption around 474,000 years ago, magma had already been accumulating underground for roughly 200,000 years.

That means the volcano spent an enormous amount of time building hidden magma reservoirs before any eruption reached the surface.

The zircon record revealed two major eruptive cycles separated by a remarkable pause.

During the first cycle, Methana produced multiple eruptions over tens of thousands of years. Then activity stopped entirely. For more than 100,000 years, no eruptions occurred.

Yet during this quiet period, zircon growth reached its highest levels.

That finding stunned researchers because zircon formation signals active magma movement and cooling underground. Instead of shutting down, the volcanic system was thriving beneath the surface.

Eventually, volcanic activity restarted about 168,000 years ago. The second eruptive cycle shifted toward different parts of the volcanic field and continued until relatively recent times. The youngest eruption occurred around 2,250 years ago and was even described by the ancient Greek historian Strabo.

The volcano’s long silence did not represent death. It represented hidden growth.

Researchers believe the answer lies in water-rich magma.

Methana sits above a subduction zone where oceanic crust sinks beneath the Earth. As this crust descends, it releases water and sediments into the mantle below. This process changes the chemistry of the mantle and helps generate unusually wet magma.

The team found evidence that Methana’s magma contained extremely high water content, sometimes exceeding 6% dissolved water by weight. Researchers call these “superhydrous” melts.

As this magma rose upward through the crust, pressure decreased and water began separating into bubbles. That triggered rapid crystallization.

The more crystals formed, the thicker and stickier the magma became.

Computer models showed that this process dramatically slowed magma ascent. In some cases, magma velocity dropped by factors between 100 and 1,000. Once crystal content exceeded roughly 30%, the magma became too rigid to rise efficiently.

Instead of erupting, it stalled underground and accumulated inside magma chambers.

Paradoxically, stronger magma supply deep underground may have produced fewer eruptions because the magma became too water-rich and crystal-heavy to escape.

“We actually believe that many subduction zone volcanoes might be periodically fed by particularly wet primitive magma, something that the scientific community has not yet fully recognized,” said lead author Răzvan-Gabriel Popa, a volcanologist at ETH Zurich. “Methana is a great example where we have seen this effect clearly, but the impact of our findings can be generalized and widespread.”

The findings carry serious implications for volcanic hazard assessment worldwide.

Many volcanoes that have remained quiet for tens of thousands of years receive little monitoring because scientists often classify them as extinct. Methana shows that this assumption can be dangerously misleading.

A volcano may appear calm while quietly storing massive amounts of magma underground. Over time, that hidden buildup could increase the potential for future eruptions.

The researchers say silent magma accumulation may occur in volcanic regions across Greece, Italy, Indonesia, Japan, the Philippines, North America and South America.

“For volcano hazard authorities, for example, in Greece, Italy, Indonesia, Philippines, South and North America, Japan, etc. this means re-evaluating the threat level of volcanoes that have been quiet for tens of thousands of years but show periodic signs of magmatic unrest,” Bachmann said.

Scientists say warning signs can still appear even during dormant periods. Earthquakes, ground swelling, gas emissions and subtle gravity changes may reveal magma recharge happening below the surface.

Modern monitoring systems can help detect these hidden processes before they become dangerous.

The Methana study also changes how scientists think about volcanic evolution.

Traditionally, long quiet periods often suggested that a magma system was cooling and dying. But the new research shows silence may instead reflect underground reservoir growth.

Researchers found that isotopes provided key clues. During Methana’s long pause, isotopic signatures suggested strong input from recycled crustal materials carried down by the subducting plate. This supported the idea that fresh, water-rich magma continued entering the system.

Over time, repeated injections of superhydrous magma may create increasingly large underground reservoirs. Scientists believe this process could eventually contribute to larger eruptions or even caldera-forming volcanic systems.

The study suggests volcanoes may spend far more time building underground than erupting at the surface.

This research could reshape how scientists and governments evaluate volcanic risk around the world. Volcanoes once considered extinct may require renewed monitoring if they show subtle signs of underground magma activity. That could improve early warning systems and help protect nearby communities.

The findings also highlight the importance of long-term geological records. Tiny zircon crystals preserved evidence that could not be seen at the surface. Similar methods may help scientists uncover hidden activity at other dormant volcanoes.

Understanding superhydrous magma systems may also improve computer models used to forecast eruptions. Researchers now know that water-rich magma can stall underground instead of erupting immediately. This insight may help scientists better estimate how magma reservoirs grow over time.

For millions of people living near volcanic regions, the message is both sobering and important: a quiet volcano is not always a safe volcano.