This summer, the 235-foot research vessel Marcus G. Langseth set out into the ocean off the Pacific Northwest. Trailing the ship were four electronic serpents, each five miles in length. These cables were adorned with scientific instruments able to peer into the beating heart of a monster a mile below the waves: Axial Seamount, a volcanic mountain.

The ship’s crew had one overriding imperative: Do not let the cables get tangled.

If they did, “it’s game over,” said Sam Mitchell, a submarine volcanologist who joined the voyage.

The ship belongs to the National Science Foundation, and is operated by Columbia University’s Lamont-Doherty Earth Observatory. Scientists aboard spent 33 days in July and August hoping to create 3D maps of the magmatic ponds and pathways in an individual, active submarine volcano for the very first time. If the researchers succeeded, they would provide a view of a hyperactive volcano that had never been seen.

Charting Axial’s internal anatomy also would improve scientists’ understanding of underwater volcanoes all over the world, most of which still lie waiting to be discovered in the gloomy depths.

Toward the end of the Langseth’s month at sea, what seemed like a nightmare began when one of the cables broke. Scientists deep within the hull of the ship, in a room full of screens that recorded the data streaming in, saw several monitors turn black. The cable’s GPS data indicated it was definitely not where it was supposed to be.

For all the scientists knew, it could be lost at sea, gone forever.

Compared to expeditions to volcanoes on land, those like the Marcus G. Langseth’s take years to prepare and have “several more zeros” added to their price tags, said Dr. Mitchell. Losing an entire family of probes and sensors to the Pacific would have been a stressful and costly accident.

“You get one shot to do these expeditions,” he said.

All eyes on Axial

The Axial Seamount sits 300 miles off the coast of Oregon. Scientists have long had hints of its vast scale, but following sonar work by the National Oceanic and Atmospheric Administration in the early 1980s, “the light bulb went off: wow, there’s a big volcano out here,” said Bill Chadwick, a seafloor geologist at Oregon State University’s Hatfield Marine Science Center, who was not involved with the expedition.

It is far from the world’s largest volcano, but the walls of the horseshoe-shaped caldron at Axial’s peak are as high as the pillars of the Golden Gate Bridge. The volcano’s main magma reservoir is two-thirds of the length of Manhattan, the same width, and taller than any building in the city.

Axial is also, volcanologically speaking, no shrinking violet.

Over geological time, a stationary mantle plume below the shifting Pacific tectonic plate has created a 1,120-mile-long line of submarine volcanoes, known as the Cobb-Eickelberg seamount chain. Axial, the youngest member of the chain, is currently sitting atop that hot spot.

The volcano also sits astride the mid-ocean ridge separating the Pacific plate to the west and the Juan de Fuca plate to the east. These plates are moving apart. Ridges like this are the birthplace of oceanic crust; molten rock rises from deep within the Earth to the seafloor, creating profuse volcanic activity.

Axial is remote and deep enough that it is vanishingly unlikely to ever cause anyone harm, said Ken Rubin, a volcanologist at the University of Hawaii at Manoa. But a better comprehension of Axial will help blunt hazards at other volcanoes that do pose risks. These include Hawaii’s Kilauea volcano, a veritable lava factory near plenty of people, and Anak Krakatau in Indonesia, which has shown how volcanoes growing out of the sea can trigger deadly mega-tsunamis.

Axial’s hyperactivity and proximity to the mainland make it one of the most comprehensively researched and monitored submarine volcanoes in the world. It has been stared at by ships, visited up close by humans in submersibles and autonomous robot divers, and since late 2014, continuously monitored by an underwater observatory network known as the Cabled Array.

The devil’s in the details

Axial’s surface has been thoroughly studied, but what lies beneath remains far more ambiguous, said Annie Kell, a seismologist at the University of Nevada, Reno, who was part of the research effort.

The only way to make sense of what happens when a volcano erupts is to peer inside. Earlier seismic scanning and listening to geological tremors have produced 2D cross-sections of parts of Axial, pinpointing key features like its faults, conduits, and primary and smaller magma reservoirs.

Like physicians, volcanologists would be better placed to understand Axial if all of its volcanic organs, magmatic veins and geological bones could be precisely imaged and placed in true 3D — which is easier said than done.

Key to the mission was a collection of pneumatic air guns, whose barrages of pressurized air created acoustic pulses. These pulses bounced around inside Axial before coming back to one of the many receivers on those cables, each of them drifting far from the ship’s own noise so as to obtain accurate readings.

Those waves migrate through the subsurface differently depending on the properties of the rock they encounter. This behavior allows scientists to work out what is present within Axial, as well as how molten or solid each of its magmatic organs are. And with the Marcus G. Langseth’s more expansive and heavily equipped array of sensors, scientists would get their 3D view into an active submarine seamount for the first time.

Praying to Poseidon

As the vessel orbited above Axial, 50 scientists, students, technicians and crew members made sure everything was going according to plan. “It’s a small city, in a way,” said Dr. Arnulf.

Data streamed in, but scientists had to take shifts watching the screens to make sure that it continued uninterrupted. It was a bit like a less thrilling version of watching the screens of green code in “The Matrix.”

“You barely see the sun, because you’re often in the hull of the boat,” said Dr. Arnulf. Spacing out was to be expected from time to time.

Everyone played their parts, but luck wasn’t always on their side. That snapped cable, perhaps caused by the tremendous physical strain it and the other components of the ship’s sensors were often under, provided an unwelcome dose of jeopardy.

Fortunately, the cable was found clinging on, somehow still attached to the ship. Because it could not be repaired at sea, the team had to finish the mission with just three cables.

At another point, the engine decided to throw a fit, requiring the team to power down the entire ship and spend the next 18 hours or so carefully reeling the four 55-ton cables back onto the vessel. The engine was fixed within an hour, but unspooling the lines again required another day. That stole two days from the cruise.

Dr. Kell, choosing to stay back on land with her children, gave mission support to the Langseth and shared the crew’s moments of technological peril. With these sorts of expeditions, so much is on the line that, she said, “you have to channel an inner peace, even though your nerves are like, oh my gosh, this is all about to go out the window!”

Dr. Arnulf was more nonchalant. “I don’t think I’ve ever been on a cruise where everything goes smoothly,” he said. “You’re always losing instruments.”

For most of the students, it was the first time at sea, so it was important to keep them entertained, said Dr. Mitchell.

Plenty of bets were wagered on ludicrous things, like how many eggs were brought on board (2,880) or how many springs were in a single air gun (24). Between shifts, people completed theses, wrote papers, read books. Dr. Arnulf, training for extreme sporting events on land, spent a fair amount of time in the gym.

Curiosity killed the cruise

Technical hitches weren’t all the team had to worry about. Local wildlife, such as fin whales, dolphins, sharks and sunfish had the potential to scupper the expedition.

Officers on deck kept an eye out for aquatic interlopers while using hydrophones to listen underwater. Marine mammals are dependent on acoustic communications, so if any got within 3,300 feet of the ship, the booming seismic equipment had to be shut down.

“You are basically creating a sound in the ocean every 15 seconds, and it’s a big sound,” said Dr. Mitchell.

Younger critters triggered a shutdown of all the equipment if they were seen at any distance. To an infant blue whale, the pulses made by the ship’s array would be like screams in its ear.

Those officers, understandably, had a lot of science-stopping power. Many things could jeopardize the mission, but Dr. Mitchell said it was surreal to think that half a decade of preparation could be nixed by a persistently curious baby whale refusing to leave the ship’s side.

Stitching together a masterpiece

Despite a few moments of chaos, the voyage achieved its objective. With the expedition concluded, scientists are now digging into all of the Langseth’s seismic slices and stitching all the data together to form a proper 3D view of Axial’s guts.

It is already clear that the Langseth’s data has game-changing potential.

The roofs of the primary and secondary magma chambers can be clearly seen in three dimensions. Their complexities are becoming clear: multiple horizontal wafers of magma, known as sills, streak through the subsurface. A previously discovered field of hydrothermal vents, some as high as buildings, has been found sitting above a newly identified third magma cache.

As they learn more about what the crew of the Langseth found, scientists stand to better understand other volcanoes, particularly those hidden beneath the sea. “A significant fraction of Earth’s volcanism happens at places like Axial,” said Dr. Rubin, referring to the mid-ocean ridges, which collectively represent a spine of volcanism stretching about 40,000 miles around the world.

But it won’t be a breeze to finish this work. Years of processing and analysis lies ahead.

“There really is both a science and an art to processing and interpreting seismic data,” said Jackie Caplan-Auerbach, a seismologist and volcanologist at Western Washington University. Compared to 2D profiles, “3D seismic data is an order of magnitude more challenging.”

The mission’s data might also help scientists better understand why Axial seems to be breathing.

When magma is rising to the surface, volcanoes tend to inflate, and Axial is no exception. Using a special arrangement of pressure sensors beneath the waves, Dr. Chadwick and his colleagues found that “if Axial’s not erupting, it’s reinflating.”

Right after one eruption ends, the volcano immediately begins refueling for the next one, getting to roughly the same level each time before it blows its top.

This rhythm allowed these scientists to predict the timing of its two most recent eruptions with ever-increasing precision. “It seems fairly well behaved, at least so far,” said Dr. Chadwick.

The next eruption is predicted to be in 2020 or 2021. Whether or not scientists achieve this forecasting hat trick, these cycles of inflation and eruption will make more sense as Axial’s magma caches come into focus.

Surface deformation is one of the main ways in which volcanoes of all kinds are monitored, from Washington State’s explosive Mount St. Helens to Hawaii’s effusive Mauna Loa. With a more holistic model of Axial and its balloon-like behavior, scientists may better understand or identify the precursors of eruptions at these volcanoes, too.

Dr. Arnulf said that members of the public can ask how an expedition to a volcano far from anyone benefits society in terms of financial gain or hazard mitigation. But, he said, anyone raising these questions might as well ask why astronomers bother studying the stars.

For him and his colleagues, gazing into the Hadean labyrinths of a restless underwater volcano holds another, more visceral appeal.

“It’s freaking awesome,” he said.