Researchers have discovered a vast plume of iron and other nutrients, more than 600 miles long, in the South Atlantic Ocean. The finding challenges long-held beliefs about the sources and abundance of undersea iron.

The discovery emerged from water samples collected in a 2007 expedition to map the chemical composition of the ocean between Brazil and Namibia. Analyzing those samples in a lab, researchers from the Woods Hole Oceanographic Institution and the University of Liverpool discovered very high levels of iron and manganese along the Mid-Atlantic Ridge, an underwater band of mountains and valleys where the earth’s tectonic plates are slowly moving apart.

“That was a real surprise for us,” said Mak A. Saito, an associate scientist at Woods Hole and the lead author of the study, which was published online by Nature Geoscience.

Researchers have long assumed that the Mid-Atlantic Ridge, and other slow-spreading ridges like it, were poor in iron. Previous studies have shown that such ridges emit very little helium, which is thought to indicate the presence of iron. The new findings cast doubt on those assumptions.