Scientists say seismic waves from earthquakes have given them a new way to look deep inside the Earth, where they've discovered our planet's inner core -- long thought to be solid iron -- has a smaller core of its own inside.

The "outer" inner core, about the size of the moon, changes its structure about halfway in, U.S. researchers working with Chinese colleagues say.

A team at the University of Illinois, working with scientists at Nanjing University, has utilized earthquake-reading technology in a novel way to plumb the mysteries of our planet's interior.

They've recorded the seismic waves that ripple through the Earth after an earthquake, making our planet "ring like a bell."

"The basic idea of the method has been around for a while, and people have used it for other kinds of studies near the surface," says Illinois geology professor Xiaodong Song. "But we are looking all the way through the center of the Earth."

What the researchers have seen there is a distinct and separate inner-inner core, about half the diameter of the whole inner core, they report in the journal Nature Geoscience.

In a further surprising discovery, iron crystals in the outer layer of the inner core are aligned north and south, while the inner-inner core's iron crystals point roughly east and west.

The crystals of the inner-inner core also behave differently, suggesting that the smaller core may be made up of a different form of iron crystal or in a different phase, the researchers say.

"The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving," Song says, suggesting tectonic changes early in the Earth's history led to these different structures in the core.

"For example, over the history of the Earth, the inner core might have had a very dramatic change in its deformation regime. It might hold the key to how the planet has evolved."

Geologists have long assumed the Earth is composed of three layers; the crust, the mantle of hot magma, and the core.

The core is also layered, with an outer core of molten liquid iron and -- it's always been believed -- an inner core of solid iron.

The new discovery of an "inner-inner" core will require some re-writing of the textbooks.

"We are right in the center -- literally, the center of the Earth," Song says of the new finding.