We do not see them erupt, yet more than half of the Earth's crust can be attributed to their dramatic explosions.
It sounds almost like a riddle. But when you understand the facts, the truth might be even more surprising.
The remnants of hundreds of thousands of deep-sea eruptions lie on the ocean floor, deep below the surface of the water.
"Beyond a basic understanding we know very little about these volcanic processes," says Isobel Yeo of GEOMAR's Helmholtz Institute for Ocean Research Kiel.
Despite our ignorance, almost 70% of the Earth's crust is believed to be produced at mid-ocean ridges such as the Mid-Atlantic Ridge (see below). These are the places where tectonic plates move away from each other.
As they do so, magma can erupt into the space, forming new crust where the plates once were. These processes are so powerful that they form enormous volcanoes, similar to Iceland's 2014-2015 Bardarbunga volcano eruption.
In fact, the mid-ocean ridges form the largest volcanic systems on Earth. But as they are largely hidden from sight, they have long remained elusive. That is something Yeo hopes to change.
"We have very little idea about the frequency and style of volcanic eruptions on the seafloor, nor the detailed structure of the crust they produce, so basically we don't really know what the crust of a large part of the Earth looks like."
That is why Yeo and colleagues have been collecting detailed images of the sea floor, 700-2,000m (2,297-6,562ft) down.
To reconstruct sub-surface volcanic history, she developed a new method to date lava flows using "hydroacoustic properties". This involves analysing how much sound the lave flows reflect back when they are hit with sonar.
Her findings, published in April 2016, revealed that these enormous eruptions occurred in the last 4,000 years. In geological terms this makes them baby volcanoes.
Yeo and colleagues returned to this site at the North Kolbeinsey Ridge, about 311 miles (500km) off the north coast of Iceland, in July 2016.
Using a torpedo-like underwater autonomous robot, they were able to capture thousands of high-resolution photos of the seafloor, never seen in such detail before. The pictures, featured here, are of such high quality that the seafloor can now be surveyed in much the same way geologists do on land.
"These observations, combined with the spatial extents of the flows, mean we can work out how much lava erupted where and when," Yeo says.
The images reaffirm that the lava flows are relatively young. They also show that there are clear periods of inactivity, where no eruptions occurred for thousands of years.
More surprisingly, some of the lava appears to be missing.
Scientists can work out how much new lava is needed to replace gaps left when ridges pull apart. The amount the team observed is not enough to account for the thickness of the crust, which is 4-6 miles (7-10km) thick.
That is why the researchers now say that even larger volcanic periods "must have preceded the volcanism we image".
It is not immediately apparent when these eruptions occurred, but it is believed to be tens of thousands of years ago.
It is only recently that scientists even realised that these lava flows existed. "We didn't even accurately know the location of the ridge axis until we surveyed the area in 2012," says Yeo.
Now that they do, they can begin to find more of it. "In a very grand sense we are trying to pin down the products from which and the timescales on which oceanic crust is constructed," says Yeo.
"It's true what they say about us knowing very little about the oceans."
The images (above and below) featured are courtesy of Isobel Yeo and GEOMAR Helmholtz Institute for Ocean Research Kiel. They were taken with the Deep Survey Cam system mounted on the ROV PHOCA.
In this location, the team found evidence for low-temperature hydrothermal venting. This may be coming from a source near the cluster of anemones in the top right of the picture above.
The dark coloured material the creatures (above) are sitting on is crumbly volcanic glass, erupted from the volcano less than 100 years ago.
This old chimney is no longer active and sits on the rim of the crater of an active volcano that is over 1,000m tall. It has been colonised by anemones and other marine organisms.
This separation of these plates causes fissuring and breaking of seafloor lavas, as seen above.
These cracks or fissures (above) can be tens of metres deep, hundreds of metres wide and many kilometres long.
More vigorous hydrothermal activity can completely bury the underlying lava flows.
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