Satellites keep catching discolored water above Ahyi, an undersea volcano near the Northern Marianas

Small plumes of discolored water appeared in the ocean above Ahyi Seamount on October 16 and again on October 21, according to the latest weekly update from the U.S. Geological Survey. The volcano sits under the sea near the Northern Mariana Islands, and satellites are the main way anyone is seeing what it is doing.

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The USGS status summary reads: “Plumes of discolored water seen in satellite imagery; no significant geophysical signals.” Ahyi remains at alert level ADVISORY with an aviation color code of YELLOW, a status that is a statement about unrest, not about an eruption in progress.

Discolored water and small plumes, seen from orbit

The USGS volcano updates page carries an Ahyi entry dated October 23, 2026. It says discolored greenish water was visible drifting several hundred meters from the summit, and it ties the plumes to degassing and hydrothermal activity that is likely associated with the summit itself.

That description matters for what it leaves out. The update does not report an ash column, a steam plume breaking the surface or an explosion. It describes tinted seawater, which is the kind of change that shows up from orbit when volcanic gas and heated fluids leave the seafloor and alter the color of the water above.

The sightings are also not new. The agency’s current notice for the volcano, issued October 22, sits on top of a record that began earlier in the year, which is why the pattern reads as recurring rather than a single sighting.

How the alert level has moved this year

On July 16, 2026, the USGS Northern Mariana duty scientist raised Ahyi from UNASSIGNED to YELLOW in a volcano observatory notice. The remarks read “underwater volcanic unrest or possible eruption observed in satellite images,” and the notice relied on satellite images alone, not on seismic or hydroacoustic data.

It had gone the other way earlier. On February 23, the agency lowered the volcano from YELLOW to UNASSIGNED, stating that no signs of volcanic unrest had been observed at Ahyi since January 9, 2026. So the volcano has been through a full cycle of unrest, quiet and renewed unrest within a single calendar year.

The status has not changed this week. The ADVISORY level has stood since the July notice, and the October update records continued observations at that level rather than an escalation. The agency defines its categories on a public alert-level page, where the ADVISORY and YELLOW designations are defined alongside the agency’s other levels.

Two features of the July notice stand out. It was numbered 2026/02 in the Northern Mariana series, and it described the event as possible eruption as well as unrest, a deliberately open wording that reflects how little can be confirmed about a vent under water. The October update is more specific about the likely mechanism, which is gas and hot fluid escaping the summit rather than a surface explosion.

The sequence also shows how the agency handles a volcano with little ground truth. A single satellite scene can start a notice, and a stretch of scenes without discolored water can end one. The February lowering rested on exactly that kind of absence: no signs of unrest since January 9.

What instruments cannot see

The weekly update says no notable activity was detected in distal geophysical monitoring data over the past week. That distal data comes from far-off instruments, and the agency’s February notice noted that underwater pressure sensors near Wake Island had not detected activity from Ahyi for several weeks.

The limit of that arrangement is stated plainly by the USGS. Because there is no local real-time monitoring at the volcano, the agency says it is unable to forecast or warn of impending eruptions. For a seamount whose summit sits beneath the sea, a satellite pass and a distant sensor are the whole network.

The gap is why the weekly update repeats a caution in plain words. Quiet distal data over a given week does not prove the vent is quiet, only that nothing large enough to register across the ocean has happened. The plumes of October 16 and 21 were small, and they were noticed because the water changed color, not because an instrument tripped an alarm.

Aviation is the other audience. The YELLOW color code is the aviation designation, and satellite information for ash clouds in the region is handled by the Washington Volcanic Ash Advisory Center. The USGS publishes the Northern Mariana Islands monitoring picture on its regional status page, and notices can be delivered by subscription through the agency’s volcano notification service.

What remains unresolved is the cause of the pattern. The USGS attributes the plumes to degassing and hydrothermal activity, and it has recorded sightings in July and again on October 16 and 21. Whether those pulses mean magma is rising or a vent is simply venting is a question that satellite color alone cannot answer.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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