Earth's inner core reversed direction and is slowing down, and scientists don't know why

• Scientists have debated for years about what's going on with Earth's inner core.
• A new study offers compelling evidence that the inner core reversed direction and is slowing down. 
• The shift happened in 2010, but it's unclear what triggered it or when the core will change back.

One of the key questions plaguing Earth scientists this past decade is "What's going on down there?"