It's not chewy or chocolate cream, nor is it a raging sea guarded by giant lizards, as in the 1959 film adaptation of Jules Verne's classic A Journey to the Center of the Earth.
So here's a high school geology refresher: the Earth has an outer core and an inner core. The outer is liquid metal, and the inner core -- we thought -- was a solid ball of hot iron.
But when seismic waves pass through the inner core, they sometimes behave strangely, moving at different speeds, for instance.
"Trying to explain, though, what about iron might be causing these interesting phenomena, that's what we didn't have much information on," explains Wendy Mao, an assistant professor at Stanford.
When she and Arianna Gleason, a post-doc at Stanford, applied the same amount of pressure to iron that they think it's under at the center of the earth -- about three million times as much pressure as at sea level -- it was surprisingly soft.
"Of course, it's much harder than silly putty," Mao says. But conditions at the core are so different from the surface, it's hard to really compare it to anything here.
Mao says, knowing the iron at the core is weak helps explain the strange behavior of seismic waves, and will help scientists better understand other quirks about our planet.