It may seem fantastical to say there is a planet within Earth, but conceptually it is true. Ever since the 1990s, geophysicists have known that Earth's inner core— a ball of iron with a radius of 746 miles (more than two-thirds the size of the moon) — spins in the center of our planet at a different pace than the rest of the globe. In a sense, this separation makes the inner core a bit like a planet of its own.
"It's probably benign, but we don't want to have things we don't understand deep in the Earth."
