Why are there huge, unusual masses inside the Earth? No one is sure. By noting how earthquakes rumble through our planet's interior, humanity has discovered two deep structures that appear to have unusual temperatures and/or chemical compositions. One hypothesis holds that the superplumes are sunken debris left over from the Earth-shattering collision that created Earth's Moon about 4.5 billion years ago. A competing hypothesis is that they are graveyards for old tectonic plates that slowly slid under each other over the past few billion years. No matter their origin, the superplumes are thought to affect Earth’s surface volcanism, possibly creating, for example, island chains such as Hawaii. Also known as large low-shear-velocity provinces (LLSVPs), Earth's superplumes are visualized in the featured animation.