Thousands of feet below the ocean's surface, sudden powerful currents stir up sediments and sweep fish and shrimp along as though they were in a river, scientists have discovered.
Past sonar readings and furrows on parts of the seafloor have hinted at these currents, called storms by some researchers.
Now they have finally been experienced, off the edge of the continental shelf in the Gulf of Mexico, some 8,000 to 10,000 feet deep.
"Abyssal storms arise suddenly and sweep along the base of the escarpment. We got to witness that first hand," Ian MacDonald of Texas A&M University said Wednesday.
The powerful currents - seen by researchers during a two-week expedition involving several dives in the deep-sea submersible Alvin - have carved furrows into the seafloor.
Scientists now are working to determine what causes the storms and what implications they might have, particularly for deep sea gas and oil wells now being developed in this region.
MacDonald said the storms are massive currents nearly 2,000 feet thick, moving at 1 to 1.5 knots. A knot is 1.15 miles per hour.
While that "doesn't sound like much," MacDonald said, normal water movement in the deep oceans is less than one-tenth knot. The relatively fast-moving water presses against anything in its path with a great force.
William R. Bryant, a Texas A&M geologic oceanographer, was aboard Alvin during one dive into one of the powerful currents.
"It was first time anyone had ever been in" one of these events, he said.
Currents had been detected by instruments before and powerful ones were suspected because of the shape of the seafloor, said Bryant, who was swept along in a 1.5 knot current. He believes some reach more than 2.5 knots.
"Those are exceptional, high currents," he said. "We see large geological features, furrows, that are the result of these currents."
Bryant said the currents are a "constraint" rather than a hazard to oil and gas rigs. The current is sufficient to undermine pipelines on the seafloor, he said, so oil and gas developers will have to determine the maximum speed of likely currents and create designs to cope with them.
The area in which the storms were found has not produced gas and oil in the past but is currently undergoing development, he said.
MacDonald discussed the results of the cruise in a telephone interview from Key West, Fla., while Bryant spoke by telephone from his office at the university.
Other findings from the expedition included:
But it was the deep storms that seemed to most surprise the scientists, who had suspected their existence but not encountered them, MacDonald said.
The water movement had channeled the seafloor into miles-long gullies. The gullies "reminded me of the buttes and mesas in the Southwest," MacDonald said.
When researchers descended into a region about 180 miles south of the Louisiana coast, the submersible was caught in the current.
"The sediments were stirred up by it. They saw fish and shrimp being whipped along like in a river ... animal life was being swept away," MacDonald said.
Unlike storms in the air, the water movement was horizontal, not up and down, he said. But the mass of moving water was 2,000 feet thick.
"That's an enormous force," he said.
The cause of the storms remains a mystery and, he added, "we don't know if they occur elsewhere ... this is the first sighting."
But the current isn’t continuous, he said. A storm lasted throughout a six-hour dive, but when they went back the next day, the storm was over.
The expedition, which departed from Galveston, Texas, on Oct. 16, was Alvin's first to the Gulf in eight years. The submersible is operated by Woods Hole Oceanographic Institution.
The National Oceanic and Atmospheric Administration sponsored the expedition through its National Undersea Research Program Center at the University of North Carolina at Wilmington.
Besides Texas A&M, other organizations participating included Louisiana State University, University of South Carolina, College of William and Mary and the Department of Energy.