Deep beneath our feet, at the border between Earth's core and the mantle above, scientists are discovering a hidden world.

Deep beneath our feet, at the border between Earth's core and the mantle above, scientists are discovering a hidden world.

The Earth is akin to a rocky, molten-in-the-middle nesting doll – or perhaps a rotting, soft-centered onion, though the latter analogy affords our majestic planet much less…