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Submarine sensors from the MULTI-MAREX project (GEOMAR Helmholtz Centre for Ocean Research Kiel) detected between January and February 2025 that approximately 300 million cubic meters of magma migrated through the subsurface of the region between Santorini and the submarine volcano Kolumbo, north of Santorini, Greece. The result was a record seismic swarm, with over 28,000 tremors in just a few weeks.
According to the GFZ Helmholtz Centre for Geosciences, the magma stagnated 4 kilometers below the Aegean seabed. In parallel, the ground in the region sank about 30 cm in a single day at specific points, according to satellite InSAR measurements.
Kolumbo is located 7 kilometers northeast of Santorini. According to Volcano Discovery, its last eruption in 1650 generated a tsunami that killed about 70 people on neighboring islands and is still a reference for risk scenarios in the region today.
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The figures from the Kolumbo seismic swarm, according to GEOMAR, GFZ, and the Greek Earthquake Planning and Protection Organization (OASP), tell the story in five points:
- 28,000 tremors recorded between January and February 2025 in the Santorini-Anafi-Amorgos-Ios region
- 300 million m³ of magma in upward migration, intrusion 4 km below the seabed
- 30 cm of subsidence in a single day at specific points on the ground
- 1.5 km in diameter for Kolumbo’s crater, with a depth of 500 meters
- 70 deaths on neighboring islands from Kolumbo’s last eruption in 1650
What it means for magma to accumulate 4 km below the seabed
The main phenomenon was described by the international team as a “dike-sill intrusion” (sill intrusion). According to GFZ, the magma rose through the crust as a horizontal layer and settled at a depth of 4 km, without reaching the top of the volcanic edifice.
In parallel, this accumulation generated tension in the surrounding rock. As a result, thousands of smaller tremors began to propagate in a network, marking the path the magma followed from the deep chamber to the stagnation point.
The tremors were not too small. According to Discover Magazine, several reached magnitudes between 4.5 and 5.3 on the Richter scale, with epicenters between Santorini, Anafi, Amorgos, and Ios.
The GEOMAR team used artificial intelligence to detect and classify each tremor. In parallel, automatic models identified the sequence of origins, generating a 3D map of the magma’s path.
The finding was unprecedented. According to Marius Kriegerowski of GFZ, “we saw for the first time, in real time, the path of magma migration beneath an active volcanic system in the Mediterranean.”
Kolumbo, a little-known volcano with a deadly history
Kolumbo is part of a larger volcanic field of more than 26 cataloged submarine structures. According to official data, its crater is 1.5 km in diameter and 500 meters deep.
In parallel, the top of the volcanic edifice lies between 15 and 18 meters below the sea surface. Therefore, it is shallow enough to pose a direct risk to navigation and generate a tsunami in the event of an explosive eruption.
According to historical records, the 1650 eruption was violent. The ash column reached thousands of meters, toxic gases suffocated people and animals in Santorini, and the tsunami hit Ios, Sikinos, Folegandros, and even Crete.
The bottom of the current crater hosts one of the most active hydrothermal fields in the Mediterranean. In parallel, fumaroles continuously expel superheated water and gases, depositing Kuroko-type metallic ore over geological time.
According to EOS, this same hydrothermal system could accelerate future eruptions if hot ascending magma comes into contact with the water in the pressurized hydrothermal chamber.
What MULTI-MAREX and GEOMAR Helmholtz do in Santorini
The MULTI-MAREX project is an international initiative led by the GEOMAR Helmholtz Centre for Ocean Research Kiel, in Germany. According to GEOMAR, the focus is to monitor submarine volcanic systems in the Mediterranean in real time via sensors on the seabed.
In parallel, the infrastructure includes broadband submarine seismometers, pressure sensors to detect seabed deformation, and dissolved gas measurement equipment. Therefore, any chemical alteration in the water above the volcano is detected almost instantly.
According to the program, the reading from January to February 2025 was the densest ever recorded for Kolumbo. The sensors captured everything from micrometric tremors to the measured ground inflation associated with magma intrusion.
In parallel, the team complemented the marine data with satellite interferometry (InSAR), which measures ground deformation via radar. The combination revealed the 30 cm of subsidence concentrated at specific points.
As of December 2025, an international mission expanded monitoring. New sensors were installed, and reinforced cooperation with Greek universities was initiated for rapid response to future swarms.
Risk of eruption and tsunami: what scientists say
The immediate risk of eruption is considered low at this moment, according to the assessment of the Greek OASP. In parallel, the stagnation of magma at 4 km depth indicates that the pressure may dissipate without reaching the surface.
The long-term scenario, however, is different. According to the Greek study published in March 2026, even moderate eruptions of Kolumbo generate tsunamis that can reach 5 to 8 meters on neighboring islands. In Crete, the propagation would arrive in 30-45 minutes.
The Greek government declared a state of emergency in Santorini in February 2025. According to
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