What is going on underneath the Pacific Ocean? Scientists have discovered huge chunks of rock that look like the remains of submerged plates in the deep mantle underneath the western Pacific Ocean. Is there a lost world down there?

Researchers used all the different types of earthquake waves to build a high-resolution model of the Earth's interior. The model, published in Nature Scientific Reports, has revealed a number of anomalous regions in the deep mantle, including a huge area under the western Pacific Ocean where seismic waves travel slower than average. Traditionally slow seismic waves have been interpreted as areas of cold tectonic plate material, recycled into the mantle over the past 200m years. However, this location is far from plate boundaries, with no geological evidence of subduction.

Andreas Fichtner, a professor at ETH Zurich, compared scientists' surprise to that of a doctor who has been examining blood circulation with ultrasound for decades and is used to finding arteries exactly where he expects them. "Then, if you give him a new, better examination tool, he suddenly sees an artery in the buttock that doesn't really belong there."

Possible explanations include an area of ancient silica-rich material that has survived since the formation of the mantle, about 4bn years ago, or mantle movements creating an accumulation of iron-rich rocks.