Scientists find a boulder weighing 2,645,547 pounds that was washed inland by an enormous tsunami

Hidden in the greenery of southern Tongatapu sits a boulder so large that locals barely noticed it was out of place. Geologists now say this limestone giant, known as Maka Lahi – Tongan for “Big Rock” – is the heaviest cliff-top boulder ever recorded.

The rock’s massive presence stands as silent proof that a powerful and devastating tsunami slammed into Tonga approximately 7,000 years ago.

Giant rock found inland

University of Queensland PhD researcher Martin Köhler stumbled on the find almost by chance. “We had been surveying the southern side of the island of Tongatapu, looking along the coastal cliffs for evidence of past tsunamis,” Köhler said.

Late one afternoon farmers pointed him inland toward an overgrown rise more than 200 meters from the sea.

“I was so surprised; it is located far inland and outside of our fieldwork area and must have been carried by a very big tsunami. It was quite unbelievable to see this big piece of rock sitting there covered in and surrounded by vegetation,” he added.

Köhler’s team used drone imagery and laser scans to build a 3D model. The block measures approximately 14 meters long, 12 meters wide, and nearly 7 meters high.

With these dimensions, researchers estimate it weighs around 1,180 tons – making it heavier than three Boeing 747 jets.

Chemical fingerprints match the coral reef limestone that forms the nearby coastline, pinpointing its original perch on a cliff over 30 meters above the sea level.

Monster tsunami vs. massive boulder

How do you budge such a monster? Numerical wave-flume experiments show that an ordinary storm – even a Category 5 cyclone – lacks the muscle.

The only scenario that works is a landslide-generated tsunami producing a single wall of water at least 50 meters high and lasting 90 seconds.

That torrent would have smashed the boulder off its ledge, hurled it inland at velocities topping 20 meters per second, and deposited it 39 meters above today’s sea level – exactly where it rests.

How old is the tsunami boulder?

Uranium-thorium dating of flowstone on the boulder’s shaded face places the event at about 6,900 years ago, early in the Holocene epoch.

Co-author Annie Lau, a coastal geomorphologist at the University of Queensland, links the discovery to Tonga’s long geological record.

“Tonga’s most recent tsunami in 2022 killed six people and caused a lot of damage. Understanding past extreme events is critical for hazard preparation and risk assessment now and in the future,” she said.

“The findings on the Maka Lahi boulder are the evidence of a tsunami in the Pacific region in the Holocene epoch which began around 11,700 years ago.”

Tsunamis leave clues

The analysis enhances our understanding of how waves move rocks, helping improve coastal hazard assessments in tsunami-prone areas worldwide.

Radiocarbon evidence suggests the wave may have been triggered by a massive submarine landslide or volcanic flank collapse along the nearby Tofua arc – processes still active today.

In 2022, Hunga Tonga’s eruption sent shockwaves worldwide and triggered nearly 20-meter-high tsunami waves.Maka Lahi demonstrates that the region can generate tsunamis more than twice that size.

Folklore meets physics

Local legends describe the demigod Māui hurling rocks across the sea while chasing birds. Those stories now appear to preserve cultural memory of events that science is only beginning to quantify.

Another megaboulder, Maui Rock, sits on Tongatapu’s western shore and was likely thrown inland by a 15th-century tsunami.

Together, the two stones bracket Tonga’s history of rare but catastrophic waves.

Future tsunami threat

Most Tongatapu residents live on the coastal plain. Present evacuation maps assume worst-case run-ups of about 20 meters. A 50-meter wave would overtop those safe zones and inundate the capital, Nuku‘alofa.

Updating hazard models will require incorporating low-probability, high-impact events like the one that moved Maka Lahi.

Early-warning systems can detect earthquake tsunamis minutes after they start, but landslide-triggered waves leave little time for alerting the public.

That makes long-term education and clearly marked evacuation routes crucial. Knowing the maximum historical run-up helps engineers decide how high to build protective berms and where to locate critical infrastructure.

What the tsunami boulder tells us

Sea-level rise increases the reach of any future tsunami. Maka Lahi moved when sea levels matched today’s, meaning future tsunamis could reach even farther as seas rise.

As climate change intensifies coastal erosion and triggers more landslides, the frequency of extreme waves could rise.

The study holds broader significance beyond Tonga, since massive cliff-top boulders also appear along coastlines from the Mediterranean to the Caribbean.

Few, however, have undergone the precise dating and flow modeling used on Maka Lahi. Its remarkable size, elevation, and inland position make it a critical reference point for improving global tsunami modeling.

Big rock with a bigger message

Standing in the brush beside the limestone behemoth, Köhler reflected on the chain of events that unveiled it: “We made a 3-D model and then went back to the coast and found the spot the boulder could have come from, on a cliff over 30 meters above the sea level.”

What seemed like a farmer’s curiosity turned out to be a deep-time archive of ocean power.

The rock offers a stark reminder that even the most tranquil coastlines have faced tsunami waves powerful enough to reshape the ground beneath our feet.

Understanding that history is not merely an academic exercise; it is a prerequisite for surviving the next colossal surge.

The study is published in the journal Marine Biology.

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