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The Turtle Pits site on the mid-Atlantic Ridge, consisting of two sulfide mounds and a black smoker chimney. Credit Credit Center for Marine Environmental Research/University of Bremen, Germany

Picture a volcano. Now imagine that its main vent extends in a line. Now imagine that this line is so long that it runs for more than 40,000 miles through the dark recesses of all the world's oceans, girding the globe like the seams of a baseball.

Welcome to one of the planet's most obscure but important features, known rather prosaically as the midocean ridges. Though long enough to circle the moon more than six times, they receive little notice because they lie hidden in pitch darkness. Oceanographers stumbled on their volcanic nature in 1973. Ever since, costly expeditions have slowly explored the undersea world, which typically lies more than a mile down.

The results can make the visions of Jules Verne seem rather tame.

The ridges feature long rift valleys and, down their middles, giant fields of gushing hot springs that shed tons of minerals into icy seawater, slowly building eerie mounds and towers that can be rich in metals like gold and silver. One knobby tower in the Pacific Ocean, nicknamed Godzilla, grew 15 stories high. Thickets of snakelike tubeworms and other bizarre creatures often blanket the hot features, as do hungry prowlers such as spider crabs.

The riot of life coexists with springs hot enough to melt lead or the plastic windows of mini submarines. With extreme care, humans and robots have measured temperatures as high as 780 degrees.

To date, the studies have been episodic. Ridge expeditions venture out fitfully, their schedules determined by fickle weather and budgets, not to mention the vagaries of crew and gear availability.

Now, scientists have inaugurated a major new effort. Off the West Coast, they have wired up a highly active ridge with hundreds of sensors and cameras, as well as cables that flash the readings to shore. The ocean observatory is to operate for at least a quarter century, replacing sporadic glimpses with continuous scrutiny.

This month, the surge of data is hitting the Internet. Hundreds of scientists around the globe will now be able to monitor one of Earth's most restless and enigmatic features as effortlessly as reading their email.

"We're seeing it come alive," said Maya Tolstoy, a marine geophysicist at the Lamont-Doherty Earth Observatory of Columbia University. She recently got a preview that included an eruption. "It's exciting," she added. "We're just starting to understand what's going on."

John R. Delaney, an oceanographer at the University of Washington who conceived of the observatory decades ago, said it would help scientists better grasp not only the volcanic ridges but the surrounding waters, which cover most of the planet.

"Suddenly, a technological door has opened on studying the ocean from within," he said in an interview. The new perspective, he added, "is the only way we're ever going to understand its true complexity - the hundreds of processes."

A main question is to what extent the volcanism changes over time. The old idea was that the eruptions of oozing lava and related activity occurred at fairly steady rates. Now, studies hint at the existence of outbursts large enough to influence not only the character of the global sea but the planet's temperature.

Experts believe the activity may carry major repercussions because the oceanic ridges account for some 70 percent of the planet's volcanic eruptions. By definition, that makes them enormous sources of heat and exotic minerals as well as such everyday gases as carbon dioxide, which all volcanoes emit.

"It's a whole new perspective on how the Earth works," said Daniel J. Fornari, a senior scientist at the Woods Hole Oceanographic Institution on Cape Cod, Mass. "We've got our eyes and ears on a part of the seafloor that's really dynamic."

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The Nature Tower, part of Lost City on the mid-Atlantic Ridge. Credit University of Washington/University of Rhode Island/National Oceanic and Atmospheric Administration, Ocean Exploration Trust

The source of all of this activity is the slow churning of Earth's molten interior, which continually rearranges the planet's two dozen or so large crustal plates. The volcanic ridges mark the places where oceanic slabs slowly pull apart, giving molten rock and gases an escape route.

The first volcanic hints came to light in 1973 when mini submarines dived on the mid-Atlantic Ridge. It runs for nearly 10,000 miles, making it the planet's longest mountain chain. The French-American team expected to see the rocky folds and fissures typical of regions on land where plates pull apart, known as divergent boundaries. Instead, they found beds of hardened lava.

The excitement rose in 1977 when an American submersible off the Galápagos Islands dived to a deep ridge. Over a hydrophone, from the seabed, a puzzled scientist told the mother ship that the volcanic site bore abundant life - contrary to the usual desertlike portrayals of the deep sea.

"There's all these animals down here," the expert reported. The unexpected fauna included red shrimp, brown mussels, pink fish with undulating tails, and dense stands of tubeworms with bright red plumes.

In the 1980s, scientists found that the hot vents could discharge giant plumes of warm, buoyant water. Zooplankton - clouds of tiny sea creatures - turned out to flourish in the mineral-rich plumes. The tracking of whale calls suggested that the giant mammals fed on the dense swarms.

Last year, a more basic discovery came to light. A team of 11 scientists reported that the blistering hot springs act as global recycling centers that turn complex carbon from ages of deceased oceanic life into much simpler chemicals that can form new organisms.

"They replace it with material that's biologically reactive," said Jeffrey A. Hawkes, a marine chemist at the University of Oldenburg in Germany, who led the research. "They're the lifeblood of the deep sea."

A triangular array that was to be deployed to measure the temperature of a venting hot spring below the surface. Credit University of Washington/National Science Foundation-Ocean Observatories Initiative/Canadian Scientific Submersible Facility

Starting in the 1990s, oceanographers received a glimpse of what continual monitoring had to offer when the Navy shared its long-secret network of undersea microphones, used during the Cold War to track enemy submarines. Suddenly, marine scientists could hear the volcanic eruptions and study their aftermath.

Recently, Dr. Tolstoy of Columbia University drew on such acoustic data from nine seabed eruptions over nearly two decades to paint a group portrait full of surprises. It turned out that all of those eruptions, from the Pacific, Atlantic and Arctic Oceans, took place from January to June.

The cause, she proposed, is Earth's slightly elliptical orbit around the sun. That changes the strength of the sun's gravitational pull on Earth during the year and, as a result, the magnitude of the tides that squeeze the planet. She said the eruptions coincided with the annual letup of the squeeze. More boldly, Dr. Tolstoy suggested that such mechanisms might help explain how the planet's regular ice ages end so abruptly - long a mystery.

Ocean levels fall sharply in such bitterly cold periods as water is tied up in massive continental ice sheets. In a paper, she proposed that the reduced pressure on the ridges might let them erupt far more frequently. As a result, more carbon dioxide would spew into the ocean and, eventually, into the atmosphere, trapping more heat and warming the planet.

In short, according to this hypothesis, the ice sheets would eventually grow large enough to initiate their own destruction, refilling the oceans. It was a radical idea that has stirred debate.

In an interview, Dr. Tolstoy said mounting evidence from the seabed suggested that the volcanic ridges were "exquisitely sensitive" to slight changes in stress, making them open to a variety of celestial influences. Scientists say such factors might one day enhance their understanding of why Earth's climate has varied so markedly over the ages, improving their computer models and forecasts.

The undersea observatory, by scrutinizing hundreds of ridge features, promises to help scientists address such riddles.

It sits atop the Juan de Fuca Ridge. The volcanic spreading center - more than 300 miles long - lies in a slanted line off the West Coast, from British Columbia to Oregon. The observatory is divided into two parts. Canada operates the northern one and the United States the southern one, part of a larger program known as the Ocean Observatories Initiative.

All told, it cost roughly $500 million - far less than the next generation of optical telescopes under construction around the globe. The National Science Foundation, the federal government's big funder of basic science, paid for the American part.

Together, the two sites feature more than 1,000 miles of cables, dozens of junction boxes and hundreds of sensors.

Instruments on the seabed include tilt meters, cameras, seismometers, temperature gauges, hydrophones, chemical probes, pressure sensors and fluid samplers. Also, mobile platforms crawl up and down long moorings to take readings higher in the water column. The observatory's main cables run ashore at Port Alberni, on Vancouver Island, and Pacific City, Ore.

"We have the most advanced cabled observatory on any volcano in the world's oceans," said Deborah S. Kelley, a scientist at the University of Washington who directs the American segment. "There'll be lots of discoveries."

Dr. Kelley joined Dr. Fornari of Woods Hole and three other marine scientists to compile a photographic atlas that summarizes what scientists have learned so far about the hidden world.

" Discovering the Deep," published in May by Cambridge University Press, is filled with hundreds of images of alien creatures as well as volcanic towers belching clouds of superheated water rich in metals and minerals. It profiles more than a dozen hot spots around the globe, including those on the Juan de Fuca Ridge, home to Godzilla. Oceanographers, the book says, have discovered vast swarms of unusually sturdy microbes thriving in dark volcanic waters as hot as 250 degrees - hotter than most boiling water on land.

Looking to the future, the authors describe the observatory and its importance for seeing the ocean from within. The investigations, they conclude, "are still in their infancy."

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Picture a volcano. Now imagine that its main vent extends in a line. Now imagine that this line is so long that it runs for more than 40,000 miles through the dark recesses of all the world's oceans, girding the globe like the seams of a baseball.

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Biggest Volcanic Bangs

History has seen some monstrous eruptions of volcanoes, from Mount Pinatubo's weather-cooling burp to the explosion of Mt. Tambora, one of the tallest peaks in the Indonesian archipelago.

The power of such eruptions is measured using the Volcanic Explosivity Index (VEI) a classification system developed in the 1980 that's somewhat akin to the magnitude scale for earthquakes. The scale goes from 1 to 8, and each succeeding VEI is 10 times greater than the last.

There haven't been any VEI-8 volcanoes in the last 10,000 years, but human history has seen some powerful and devastating eruptions. Because it's extremely difficult for scientists to be able to rank the strength of eruptions in the same VEI category, here we present the 10 most powerful volcanoes within the last 4,000 years (within human records) first in order of strength, then within each category, in chronological order.

But let's start with a supervolcano eruption surprisingly close to home, registering a magnitude-8, from our distant past.

1. Yellowstone eruption, 640,000 years ago (VEI 8)

The entire Yellowstone National Park is an active volcano rumbling beneath visitors' feet. And it has erupted with magnificent strength: Three magnitude-8 eruptions rocked the area as far back as 2.1 million years ago, again 1.2 million years ago and most recently 640,000 years ago. "Together, the three catastrophic eruptions expelled enough ash and lava to fill the Grand Canyon," according to the U.S. Geological Survey. In fact, scientists discovered a humongous blob of magma stored beneath Yellowstone, a blob that if released could fill the Grand Canyon 11 times over, the researchers reported on April 23, 2013, in the journal Science.

The latest of the trio of supervolcano eruptions created the park's huge crater, measuring 30 by 45 miles across (48 by 72 kilometers).

The chance of such a supervolcano eruption happening today is about one in 700,000 every year, Robert Smith, a seismologist at the University of Utah in Salt Lake City, told Live Science previously.

2. Huaynaputina, 1600 (VEI 6)

This peak was the site of South America's largest volcanic eruption in recorded history. The explosion sent mudflows as far as the Pacific Ocean, 75 miles (120 km) away, and appears to have affected the global climate. The summers following the 1600 eruption were some of the coldest in 500 years. Ash from the explosion buried a 20-square-mile (50-square-km) area to the mountain's west, which remains blanketed to this day.

Although Huaynaputina, in Peru, is a lofty 16,000 feet (4,850 meters), it's somewhat sneaky as volcanoes go. It stands along the edge of a deep canyon, and its peak doesn't have the dramatic silhouette often associated with volcanoes.

The 1600 cataclysm damaged the nearby cities of Arequipa and Moquengua, which only fully recovered more than a century later.

3. Krakatoa, 1883 (VEI 6)

The rumblings that preceded the final eruption of Krakatoa (also spelled Krakatau) in the weeks and months of the summer of 1883 finally climaxed with a massive explosion on April 26-27. The explosive eruption of this stratovolcano, situated along a volcanic island arc at the subduction zone of the Indo-Australian plate, ejected huge amounts of rock, ash and pumice and was heard thousands of miles away.

The explosion also created a tsunami, whose maximum wave heights reached 140 feet (40 meters) and killed about 34,000 people. Tidal gauges more than 7,000 miles (11,000 km) away on the Arabian Peninsula even registered the increase in wave heights.

While the island that once hosted Krakatoa was completely destroyed in the eruption, new eruptions beginning in December 1927 built the Anak Krakatau ("Child of Krakatau") cone in the center of the caldera produced by the 1883 eruption. Anak Krakatau sporadically comes to life, building a new island in the shadow of its parent.

4. Santa Maria Volcano, 1902 (VEI 6)

The Santa Maria eruption in 1902 was one of the largest eruptions of the 20th century. The violent explosion in Guatemala came after the volcano had remained silent for roughly 500 years, and left a large crater, nearly a mile (1.5 km) across, on the mountain's southwest flank.

The symmetrical, tree-covered volcano is part of a chain of stratovolcanoes that rises along Guatemala's Pacific coastal plain. It has experienced continuous activity since its last blast, a VEI 3, which occurred in 1922. In 1929, Santa Maria spewed forth a a pyroclastic flow (a fast-moving wall of scalding gas and pulverized rock), which claimed hundreds of lives and may have killed as many as 5,000 people.

5. Novarupta, 1912 (VEI 6)

The eruption of Novarupta one of a chain of volcanoes on the Alaska Peninsula, part of the Pacific Ring of Fire, was the largest volcanic blast of the 20th century. The powerful eruption sent 3 cubic miles (12.5 cubic km) of magma and ash into the air, which fell to cover an area of 3,000 square miles (7,800 square km) in ash more than a foot deep.

6. Mount Pinatubo, 1991 (VEI 6)

A stratovolcano located in a chain of volcanoes in Luzon, Philippines, created along a subduction zone, the cataclysmic eruption of Pinatubo was a classic explosive eruption.

The eruption ejected more than 1 cubic mile (5 cubic kilometers) of material into the air and created a column of ash that rose up 22 miles (35 km) in the atmosphere. Ash fell across the countryside, even piling up so much that some roofs collapsed under the weight.

The blast also spewed millions of tons of sulfur dioxide and other particles into the air, which were spread around the world by air currents and caused global temperatures to drop by about 1 degree Fahrenheit (0.5 degree Celsius) over the course of the following year.

7. Ambrym Island, 50 AD (VEI 6 +)

The 257-square-mile (665-square-km) volcanic island, part of the Republic of Vanuatu, a tiny nation in the southwestern Pacific Ocean, witnessed one of the most impressive eruptions in history, one that sent a wave of scalding ash and dust down the mountain and formed a caldera 7.5 miles (12 km) wide.

The volcano has continued to be one of the most active in the world. It has erupted close to 50 times since 1774, and has proved a dangerous neighbor for the local population. In 1894, six people were killed by volcanic bombs and four people were overtaken by lava flows, and in 1979, acid rainfall caused by the volcano burned some inhabitants.

8. Ilopango Volcano, 450 AD (VEI 6 +)

Although this mountain in central El Salvador, just a few miles east of the capital city San Salvador, has experienced only two eruptions in its history, the first known eruption was a doozy. It blanketed much of central and western El Salvador with pumice and ash, and destroyed early Mayan cities, forcing inhabitants to flee.

Trade routes were disrupted, and the centers of Mayan civilization shifted from the highland areas of El Salvador to lowland areas to the north and in Guatemala.

The summit's caldera is now home to one of El Salvador's largest lakes.

9. Mount Thera, approx. 1610 B.C. (VEI 7)

Geologists think that the Aegean Islands volcano Thera exploded with the energy of several hundred atomic bombs in a fraction of a second. Though there are no written records of the eruption, geologists think it could be the strongest explosion ever witnessed.

The island that hosted the volcano, Santorini (part of an archipelago of volcanic islands in Greece), had been home to members of the Minoan civilization, though there are some indications that the inhabitants of the island suspected the volcano was going to blow its top and evacuated. But though those residents might have escaped, there is cause to speculate that the volcano severely disrupted the culture, with tsunamis and temperature declines caused by the massive amounts of sulfur dioxide it spewed into the atmosphere that altered the climate.

10. Changbaishan Volcano, 1000 AD (VEI 7)

Also known as the Baitoushan Volcano, the eruption spewed volcanic material as far away as northern Japan, a distance of approximately 750 miles (1,200 kilometers). The eruption also created a large caldera nearly 3 miles (4.5 km) across and a half-mile (nearly 1 km) deep at the mountain's summit. It is now filled with the waters of Lake Tianchi, or Sky Lake, a popular tourist destination both for its natural beauty and alleged sightings of unidentified creatures living in its depths.

Located on the border of China and North Korea, the mountain last erupted in 1702, and geologists consider it to be dormant. Gas emissions were reported from the summit and nearby hot springs in 1994, but no evidence of renewed activity of the volcano was observed.

11. Mt. Tambora, Sumbawa Island, Indonesia - 1815 VEI 7

The explosion of Mount Tambora is the largest ever recorded by humans, ranking a 7 (or "super-colossal") on the Volcanic Explosivity Index, the second-highest rating in the index. The volcano, which is still active, is one of the tallest peaks in the Indonesian archipelago.

The eruption reached its peak in April 1815, when it exploded so loudly that it was heard on Sumatra Island, more than 1,200 miles (1,930 km) away. The death toll from the eruption was estimated at 71,000 people, and clouds of heavy ash descended on many far-away islands.

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The 11 Biggest Volcanic Eruptions in History

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History has seen some monstrous eruptions of volcanoes, from Mount Pinatubo's weather-cooling burp to the explosion of Mt. Tambora, one of the tallest peaks in the Indonesian archipelago. The power of such eruptions is measured using the Volcanic Explosivity Index (VEI) a classification system developed in the 1980 that's somewhat akin to the magnitude scale for earthquakes.

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A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation of LIPs is variously attributed to mantle plumes or to processes associated with divergent plate tectonics.

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The Biggest Volcanic Eruption in Human History

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Supervolcanoes MAPPED Where the world’s biggest volcanoes