Scientists have found something beneath Bermuda that changes everything we thought we knew about how volcanic islands work. Deep below the island’s turquoise waters lies a rock layer so thick and so unusual that researchers are calling it unprecedented in the history of geological study. The structure measures 20 kilometers (12.4 miles) thick, roughly twice the size of any similar layer ever documented anywhere on the planet.
The discovery comes from a team led by seismologist William Frazer of Carnegie Science and Jeffrey Park of Yale University, who published their findings in the journal Geophysical Research Letters. What they found defies conventional geological wisdom. Beneath the oceanic crust sits an unusual rock layer that is less dense than the surrounding material. This layer is not supposed to exist, at least not at this scale. Most volcanic islands have similar structures measuring between 3 and 10 kilometers (1.9 to 6.2 miles) thick. Bermuda’s measures approximately 20 kilometers (12.4 miles), making it an outlier that demands explanation.
The island itself sits on an oceanic swell where the ocean crust rises about 500 meters (1,640 feet) above the surrounding seafloor. This elevated position has persisted for more than 30 million years, long after Bermuda’s volcanoes went silent. Under normal circumstances, volcanic islands begin sinking once their heat source disappears. The tectonic plate shifts away from the deep mantle hotspot, the cooling crust contracts, and the volcano slowly subsides into the ocean. Bermuda has not followed this pattern.
Frazer explained to Live Science that after the bottom of the oceanic crust, scientists expect to find the mantle. “But in Bermuda, there is this other layer that is emplaced beneath the crust, within the tectonic plate that Bermuda sits on,” he said. The layer acts like a natural raft, its lower density providing buoyancy that keeps the island elevated above the surrounding ocean floor.
The research team used data from 396 distant earthquakes to make their discovery. These earthquakes were powerful enough to send clean vibrations through the Earth, acting like a geological X-ray that allowed scientists to paint a vertical picture of the rocks beneath Bermuda down to approximately 31 kilometers (19 miles) deep. The scientists worked with a technique called multiple-taper correlation receiver function imaging to identify the shallow structure beneath the island. They analyzed waveforms from the BBSR borehole seismic station, examining earthquakes with magnitudes greater than 5.5 occurring at distances between 30 and 100 degrees from Bermuda.
Each earthquake sent seismic waves through the planet, and when those waves encountered changes in rock density or composition, they converted from one type of wave to another. By analyzing these conversions, the scientists could determine where interfaces between different rock layers existed. The quality control applied to the data was strict. The team required signal-to-noise ratios greater than 2 in two different frequency bands and required the causal receiver function amplitude to be twice as large as the acausal amplitude. These quality checks ensured that the signals they analyzed represented real geological structures rather than noise or processing artifacts.
The measurements come from analyzing how seismic waves convert from compressional waves to shear waves when they encounter density or velocity contrasts in the subsurface. The team used a specialized technique that allows analysis at higher frequencies than traditional receiver function methods. Higher frequency analysis provides better resolution of shallow structures, allowing the researchers to distinguish between closely spaced interfaces. To test the robustness of their observations, the researchers varied several key parameters. They tested different seismic velocities for the underplated layer, and the presence of a thick underplated layer remained consistent across all tests. They also analyzed the data using different corner frequencies from 0.5 to 3.0 Hertz, and at all frequencies tested, the signal from the base of the underplated layer appeared clearly.
The team identified four distinct seismic interfaces beneath Bermuda through their analysis. The shallowest sits at approximately 3.28 kilometers (2 miles) depth and represents the base of the volcanic edifice, the structure of the island itself. Below that lies a layer about 3.95 kilometers (2.5 miles) deep that the researchers interpret as the upper portion of the oceanic crust. The third interface, located at roughly 10.8 kilometers (6.7 miles) depth, represents what scientists call the fossil Moho, the boundary between the Earth’s crust and mantle. The fourth and deepest interface marks the bottom of the massive 20-kilometer-thick (12.4-mile) underplated layer at approximately 31 kilometers (19 miles) below the surface.
Sarah Mazza, a geologist at Smith College in Massachusetts who was not involved in the research, told Live Science that material left over from Bermuda’s days of active volcanism is potentially holding the island up as an area of high relief in the Atlantic Ocean. “The fact that we are in an area that was previously the heart of the last supercontinent is, I think, part of the story of why this is unique,” she said. The location matters profoundly. Bermuda sits in a region of the Atlantic Ocean that once formed the center of Pangaea, the last supercontinent that broke apart roughly 200 million years ago. The geological history of this area includes complex processes related to continental rifting, ocean basin formation, and the recycling of ancient crustal materials back into the mantle.
The team’s analysis provides an answer to a long-standing puzzle about Bermuda. The island has remained elevated above the surrounding ocean floor for 31 million years without any apparent heat source. Traditional models of volcanic islands assume that a hot mantle plume rising from deep in the Earth provides both the magma source for volcanism and the dynamic support for the topographic swell. As the tectonic plate moves away from the plume, the volcanic features subside and the swell should gradually flatten. Bermuda has not subsided significantly since its volcanism ceased, suggesting that dynamic support from a hot plume is not necessary to maintain the swell.
The underplated layer appears to provide permanent structural support through compositional buoyancy rather than thermal effects. The team’s analysis suggests the underplated layer is approximately 50 kilograms per cubic meter less dense than the lithospheric mantle rock it replaced. This density difference, while seemingly small at about 1.5 percent, is sufficient to provide the buoyancy needed to support Bermuda’s elevated position above the surrounding ocean floor. The researchers calculated that if the underplating had a typical thickness of 5 to 10 kilometers (3.1 to 6.2 miles), a larger reduction in density would be required to generate the same topographic effect.
The absence of modern volcanism at Bermuda makes the persistent swell particularly puzzling. Unlike Hawaii, which has an active volcano and a clear age-progressive chain of islands and seamounts stretching across the Pacific Ocean, Bermuda stands alone. The last known volcanic eruption occurred 30 to 35 million years ago, and there is no evidence of a deep mantle plume currently beneath the island. Global seismic tomography, which creates three-dimensional images of Earth’s interior by analyzing how seismic waves travel through rocks of different temperatures and compositions, shows no evidence for a plume in the lower mantle beneath Bermuda.
Bermuda also shows other geophysical characteristics consistent with a thick buoyant underplated layer. The island has positive residual topography, meaning it sits higher than expected, but a negative residual gravity anomaly, meaning the gravity field is weaker than expected. A reduced-density layer of rock is consistent with this observation and may not require lithospheric thinning as previously suggested. The heat flow at Bermuda is normal, not elevated, which is consistent with the absence of a hot thermal anomaly but difficult to explain if an active mantle plume existed beneath the island.
Three potential mechanisms could explain how this thick layer formed beneath Bermuda. The first involves magma that stalled beneath the Moho, the boundary between crust and mantle, instead of erupting to the surface. Over time, this magma would have cooled and solidified, building what scientists call a mafic pluton beneath the oceanic crust. Park told Brighter Side of News that some magma may have stalled beneath the Moho instead of erupting, building this structure over time. If the magma contained unusual compositions or temperatures, it could have created a layer with properties different from typical mantle rock.
The second possibility involves volatile-rich melts rising beneath Bermuda. Park explained that they found volatile-rich melts rising beneath Bermuda could have efficiently depleted and modified the uppermost mantle, leaving behind a lighter residue. Geochemical analysis of volcanic rocks from Bermuda shows evidence that the magma source included volatile-rich recycled materials that may have been stored in the mantle transition zone, a layer between 410 and 660 kilometers (255 and 410 miles) depth where the mineral structure of mantle rocks changes. These recycled materials could have created unusual melting conditions that generated the thick underplated layer.
The third mechanism involves metasomatic underplating. Park noted another possibility is metasomatic underplating, where hot upwelling material cracks the crust, lets seawater in, and partially serpentinizes the mantle. In this scenario, seawater infiltrates downward through cracks in the oceanic crust. The seawater then reacts with iron-rich minerals in the mantle to create serpentine minerals that have lower density than the original rock. This process can create layers of intermediate seismic velocity similar to what the researchers observed beneath Bermuda.
One or more of these processes may have operated beneath Bermuda during or shortly after its volcanic activity 30 to 35 million years ago. The result is a geological structure that appears to be unique on Earth, or at least unique among the ocean islands that scientists have studied in detail. The lateral extent of the underplating remains unknown. The team’s receiver function technique provides information only beneath the seismic station on Bermuda itself. The layer could extend 50 to 100 kilometers (31 to 62 miles) from the island, or it could extend much farther.
Early active-source seismic surveys conducted in the 1950s did not have the depth coverage to image this structure, and the closest modern active-source seismic survey was conducted approximately 200 kilometers (124 miles) to the southeast and did not detect underplating beneath the oceanic crust. At other volcanic islands, scientists have detected underplating extending beyond the volcanic edifice. Beneath the Hawaiian swell, thick underplating measuring 5 to 10 kilometers (3.1 to 6.2 miles) has been observed more than 200 kilometers (124 miles) from the center of the plume track. The thickness of Hawaiian underplating decreases with distance from the plume center, suggesting that the process responsible for creating it operated most intensely directly beneath the active volcano. If Bermuda follows a similar pattern, the thick underplating detected beneath the island may thin considerably away from the volcanic edifice.
The discovery has implications for how scientists understand ocean island swells in general. If thick underplating can support a swell without requiring an active thermal anomaly, then other ocean swells might also be supported by compositional buoyancy rather than temperature effects. This distinction matters for understanding mantle dynamics, plate tectonics, and the long-term evolution of ocean basins. The research adds to a growing body of work suggesting that ocean islands display more geological diversity than traditional models suggest. Some islands show thick underplating, others show thin underplating or no underplating at all. Some islands sit on swells with active volcanism and evidence for hot mantle plumes, while others like Bermuda maintain elevated topography long after volcanism has ceased.
The thick underplating beneath Bermuda could extend the island’s elevated position indefinitely into the future. Unlike thermal support from a hot plume, which would decay once the plate moved away from the heat source, compositional buoyancy from less-dense rock provides permanent structural support. Barring major tectonic reorganization or processes that could alter the density of the underplated layer, Bermuda should remain elevated above the surrounding ocean floor for millions of years to come.
Frazer is now looking at other islands around the globe to determine whether any have similar layers to what the team discovered beneath Bermuda, or if Bermuda truly represents a unique case. “Understanding a place like Bermuda, which is an extreme location, is important to understand places that are less extreme and gives us a sense of what are the more normal processes that happen on Earth and what are the more extreme processes that happen,” Frazer explained. The research demonstrates the value of long-running seismic stations in geologically interesting locations. The BBSR station has been operating for decades, accumulating a large archive of high-quality seismic data. Modern analysis techniques applied to these archived data reveal structures that were not detectable with older methods.
The discovery also raises questions about other locations across the planet. Are there additional islands with unusually thick underplating that scientists have not yet detected? Do certain conditions favor the formation of thick underplating over thin underplating? How common is compositional support for ocean swells compared to thermal support? These questions will require additional research at other volcanic islands and oceanic swells around the planet. Scientists will continue studying Bermuda to better understand the thick layer beneath it. Additional research may reveal the lateral extent of the underplating, its exact composition and density, and the processes that created it.
Comparisons with other volcanic islands will help determine whether Bermuda is truly unique or whether other locations with similar structures have simply not been studied in sufficient detail. Each answer will generate new questions about how our planet works and how geological processes create the landscapes we see today. The implications extend beyond Bermuda. If thick underplating can form beneath volcanic islands and provide long-term structural support, then the history of ocean basins may be more complex than previously recognized. Subsided seamounts and guyots, which are flat-topped underwater mountains scattered across the ocean floor, may have had different amounts of underplating beneath them, affecting how quickly they subsided and how long they remained near sea level.
For Bermuda itself, the findings add a new chapter to the island’s geological story. The rock layer buried kilometers beneath the ocean floor is not just a scientific curiosity but the foundation that keeps the island above water. Without this massive raft of less-dense rock, Bermuda might have subsided into the Atlantic Ocean millions of years ago, just as volcanic islands typically do when their heat source disappears. The layer represents a frozen moment from 30 million years ago when volcanic processes beneath the island created something extraordinary that has persisted ever since.
The massive structure beneath Bermuda stands as one of Earth’s hidden geological wonders. Measuring more than 20 kilometers (12.4 miles) thick and extending deep into the oceanic lithosphere, this layer of modified mantle rock has kept a tiny island above water for 31 million years after its volcanoes fell silent. The discovery shows that Earth still holds secrets, even in well-studied locations, and that new analysis techniques can reveal structures that remained hidden for decades. Bermuda owes its existence to a geological accident that created the thickest underplated layer ever documented, a massive foundation unlike anything else scientists have found anywhere on Earth.
Source:
The research was published in Geophysical Research Letters by William Frazer of Carnegie Science and Jeffrey Park of Yale University: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL118279
