Olympus Mons, Mars’ massive shield volcano that rises over 20 kilometers into the air and covers an area the size of Arizona, is one of the most impressive geological features in the solar system. It’s difficult to imagine such a gargantuan volcano on the surface of the Earth. But maybe that’s only because we haven’t looked hard enough.

The Shatsky Rise is a seafloor plateau in the Pacific, about 900 miles off the coast of Japan. In total, it’s about the size of California (sticking with US states as units of measure). Like other oceanic plateaus, it was built by huge outpourings of lava. They’re thought to be similar to continental “flood basalts” like the Columbia River Basalts and India’s Deccan Traps that are associated with mantle hot spots. Prodigious amounts of lava issued forth from long fissures and smaller vents during those events. There are still things we don’t know about these kinds of eruptions, so you can imagine how much less we know about the plateaus at the bottom of the ocean.

William Sager, a Professor of Geophysics at the University of Houston, is one of the researchers who has been studying the Shatsky Rise. The plateau is made of several separate chunks called “massifs” that formed about 145 million years ago, the largest of which is Tamu Massif. “I have suspected for years that Tamu might be a single volcano,” Sager told Ars. “We have known the shape for many years, and it appears to be a big dome. But when I would make this assertion, other scientists would say that I should not be so quick to jump to that conclusion because continental flood basalts have complex eruption sources, often along long fissures. They were right in the sense that the shape alone is not proof.”

The latest project, which involved researchers from six universities, sought to learn more about the inner structure of Tamu Massif. Sediment and rock cores collected over a few years gave a glimpse of the layers of basalt that stacked up to form Tamu, but that didn’t indicate anything about where they erupted from. To investigate the source, they used seismic imaging to build cross sections through the massif. (This technique emits seismic energy using powerful airguns and records the echoes of those waves bouncing off layers at and below the surface.)

Those cross sections didn’t show a jumbled collection of vents glued together by lava flows. Instead, Tamu appears to be an orderly, layered stack of lavas that flowed hundreds of kilometers downhill from a central point. That would make Tamu a single, enormous volcano—easily the largest identified on Earth. In fact, its area is about equal to Olympus Mons. While it’s “only” about three kilometers tall, it has greatly depressed the thin oceanic crust beneath it. Combined with the fact that Olympus Mons is perched on top of crust that is quite thick, Tamu may be only 25 percent smaller by volume.

“An interesting caveat,” Sager said, “is that we do, in fact, only have 3 seismic lines that cross Tamu Massif, so there are large gaps in between (tens to hundreds of kilometers). So it is possible that we missed something big. This is the life of a marine geophysicist. Those seismic lines were hard won—they took years of planning, months to collect, and cost [the National Science Foundation] millions of dollars—so we will not be getting any more anytime soon. We often have to interpret data with gaps in the oceans.”

Though it’s now more than two kilometers underwater, cores of the sediment draped over Tamu show that its crown once neared sea level (corals have even been found)—but it probably never emerged from the waves.

If Tamu is a single volcano (a conclusion not everyone is ready to accept just yet), might there be similarly massive volcanoes lurking undiscovered in other oceanic plateaus?

“After learning that Tamu Massif is an enormous volcano, now I would not be surprised if we found some other oceanic plateau is the same. I know colleagues who are studying Ontong Java Plateau, which is the biggest, also by collecting seismic imaging data. I have not heard their findings yet, but maybe one day they will say that it is one volcanic mass or a small number of coalesced features and then [it] will be the biggest volcano. That would be cool,” Sager said.

In their paper, the researchers write, “Geophysical data from Tamu Massif demonstrate that the huge volcanoes found on other solar system bodies have cousins here on Earth. The Earth variety is poorly understood because these monsters found a better place to hide—beneath the sea.”

Nature Geoscience, 2013. DOI: 10.1038/NGEO1934 (About DOIs).