Hidden in the murky depths of the Pacific Ocean off the coast of California, a series of asphalt volcanoes rise from the seafloor. The underwater domes are like giant parking lots, teeming with life and belching methane.
The asphalt was spewed into the sea 40,000 years ago and hardened, scientists explained today. The domes are located about 10 miles (16 kilometers) off the Santa Barbara coastline of California, at the bottom of the Santa Barbara Channel.
The largest of the domes lies at a depth of 700 feet (220 meters), too deep for scuba diving, which is why they hadn't been seen until now.
"They're larger than a football-field long and as tall as a six-story building," said lead researcher David Valentine, a geoscientist at the University of California, Santa Barbara. "They're massive features, and are made completely out of asphalt."
Rather than lava, these volcanoes likely spit out petroleum and natural gas, which is made mostly of methane (a greenhouse gas). And now scientists have images of the volcanoes, showing the seafloor topography.
The discovery is detailed online this week in the journal Nature Geoscience.
Diving for volcanoes
Valentine credits Ed Keller, an earth scientist at UCSB, with guiding him and colleagues to the site.
"Ed had looked at some bathymetry [seafloor topography] studies conducted in the 1990s and noted some very unusual features," Valentine said.
Based on Keller's research, Valentine and other scientists sent the submersible robot Alvin into the area in 2007 and discovered the source of the mystery. Using the sub's robotic arm, the researchers broke off samples and brought them to labs at UCSB and Woods Hole Oceanographic Institution in Massachusetts (WHOI) for testing.
"It was an amazing experience, driving along…and all of a sudden, this mountain is staring you in the face," said WHOI director Christopher M. Reddy, who said the dome was teeming with undersea life. "It was essentially an oasis," he said, "almost like an artificial reef."
In 2009, Valentine and colleagues made two more dives to the area in Alvin, and they sent down an autonomous underwater vehicle, Sentry, to take photos of the volcanoes as it glided about 9 feet (3 meters) above the ocean floor.
"When you 'fly' Sentry over the seafloor, you can see all of the cracking of the asphalt and flow features," Valentine said. "All the textures are visible of a once-flowing liquid that has solidified in place."
Using various chemical and geophysical analyses, the research team found the structures are made of asphalt and had formed when petroleum flowed from the seafloor some 30,000 to 40,000 years ago.
Without access to sophisticated lab equipment, Reddy employed a "25-cent glass tube, the back of a Bic pen and a little nail polish remover" to analyze the crusty substance. He used the crude tools like a mortar and pestle to grind the rock, "and literally within several minutes, it became a thick oil. This immediately said to me that this was asphalt."
The volcanoes underscore a little-known fact: "Half the oil that enters the coastal environment is from natural oil seeps like the ones off the coast of California," Reddy said.
The two largest volcanoes, which are less than a mile (about a kilometer) apart, have pits or depressions surrounding them. These pits, according to Valentine, are signs of "methane gas bubbling from the sub-surface."
The discovery that vast amounts of methane once emanated from the volcanoes caused the scientists to wonder if there might have been an environmental impact on the area during the Ice Age.
"It became a dead zone," Valentine said. "We're hypothesizing that these features may have been a major contributor to those events."
While the volcanoes have been dormant for thousands of years, the 2009 Alvin dive revealed a few spots where gas was still bubbling.
"We think it's residual gas," said Valentine, who added that the amount of gas is so small it's harmless, and never reaches the surface.
- Gallery: Wild Volcanoes
- World's Deepest Undersea Vents Discovered
- 101 Amazing Earth Facts
Sign up for the Live Science daily newsletter now
Get the world’s most fascinating discoveries delivered straight to your inbox.