Figuring out anything from 115 million years ago is a challenge. But realizing that a tsunami happened is even more remarkable.

The discovery came almost by chance. Researchers were examining rocks from a quarry on Japan’s island of Hokkaido when they found evidence of an ancient catastrophe: beds of amber buried deep in the sediment of what was once a pelagic seafloor. Unlike typical amber deposits found in forests or shallow coastal environments, these fossilized tree resins were found more than 150 kilometers from ancient shorelines. They were embedded within deep marine sandstone. And this is where the story starts.

The study concludes that the amber was originally formed as resin in coastal forests. During massive tsunami events, entire sections of these forests — including trees, resin, plant debris, and soil — were violently uprooted and transported directly into the deep sea.

Resin in the deep

Tsunamis are some of the most destructive natural forces on Earth. Triggered by undersea earthquakes or landslides, they hurl massive walls of water across ocean basins and can flatten entire coastlines in minutes. But while they are well documented in modern times, they are elusive in deeper geological history.

Coastal tsunami deposits, like sand layers or shell beds, are often erased by the very turbulence they record. And in the ancient rock record, they can be hard to tell apart from other high-energy events like cyclones or floods.

But amber offers a new kind of evidence.

The team focused on a 150-meter-thick sequence of Early Cretaceous sediments (dated to roughly 116–114 million years ago). The lower layers were volcanic in origin. Sitting atop them, a thick stack of marine sediments was deposited when tectonic plates collided and pushed up new landscapes. Somewhere in this transition, the amber made its way into the mix. It shouldn’t have, but it did.

Thirty distinct amber-bearing beds were found in the marine sandstone. In five of them, amber made up to 80% of the exposed surface. One layer was so resin-rich it formed a blanket of amber up to seven centimeters thick across a ten-meter bedding plane. This alone would be unusual. But what astonished researchers was what the amber looked like.

“Flames” that signal water

The researchers used a technique called “fluorescence grinding tomography” to essentially slice and image the amber in fine layers under UV light. With this, they found that sheets of amber had folded, flowed, and even moved upward to form flame-like structures. This is usually seen when wet sand deforms under pressure.

This suggests that the amber hadn’t solidified and was still a soft, sticky resin when it flowed underwater, mingled with mud, and settled onto the deep-sea floor.

Using a novel technique called fluorescence grinding tomography — essentially slicing and imaging the amber in fine layers under UV light — they revealed that the fossil resin bore soft-sediment deformation structures. Planar sheets of amber had folded, flowed, and even formed upward “flames,” shapes usually seen when wet sand deforms under pressure.

These weren’t dry, hardened chunks tossed by waves. They were soft, sticky rivers of resin that had flowed underwater, mingled with mud, and settled onto the deep sea floor before they had time to harden.

“Modern resin hardens in about one week after exudation,” the team wrote, citing past studies. “In contrast, the resin flowing into water keeps soft unless exposed to the air”.

But it was still not clear how the amber reached the seafloor in the first place. To solve that, the team looked at the surrounding geology. In particular, they looked at what was around the amber.

Trees at the bottom of the ocean

The beds of amber weren’t alone. They came bundled with plant debris, large chunks of driftwood (some over a meter long), and “rip-up clasts” — massive fragments of seafloor mud wrenched and tossed into the sediment. All of these were interbedded with turbidites, graded sandstone deposited by underwater avalanches of sediment.

Together, the signs pointed to a single culprit: a tsunami.

The researchers propose a cascade of events. First, a massive earthquake — likely triggered by the subduction of a tectonic plate — sent a tsunami racing toward the land. It smashed through coastal forests, ripping up trees and showering the ocean with resinous sap. At the same time, a vast carbonate platform collapsed offshore, sending limestone blocks tumbling into the depths.

From both land and shallow sea, torrents of debris were flushed into the pelagic basin. Some of this debris, like sand and mud, sank quickly. Other debris — like driftwood and resin — floated longer, then settled slowly over time. The repeating nature of the amber layers — 30 separate beds over several meters of rock — suggests that such events happened more than once during this tectonically active interval.

Amber is really useful

Amber has long fascinated paleontologists for its preservation of ancient life. Previously, geologists have found insects, pollen grains, and even microscopic creatures preserved in prehistoric amber. But here, in the deep sea, amber becomes something else entirely: a recorder of tragedy. It is a window into moments when the Earth itself shook and shattered ecosystems in minutes.

What makes this discovery so scientifically important is that tsunami deposits are notoriously hard to identify in the rock record. Their signatures often blur into those of storms or floods. Yet in the pelagic environment, far from rivers or cyclones, such events stand out.

This research adds a powerful new tool to the search for ancient tsunamis — especially in places where coastlines have long since vanished. It also reframes amber as more than just a forest relic. In the words of the authors, “the novel perspective of resin as a soft-sediment unveils the whole sedimentary process from erosion to burial, a view neglected by previous sedimentological studies.”

The study was published in Nature Scientific Reports.