Research teams from the University of Illinois and Nanjing University in China used earthquake-reading technology to research the earth's core and found that it has an inner core of its own with some surprising properties.
The core of our planet is composed of a liquid outer part and a solid inner part, both of which are made up of primarily iron. According to American and Chinese researchers there is another core inside the inner core with a radius of about 1200 kilometers (which is smaller than our moon).
Studies where done by the use of vibrations caused by earthquakes, similar to using ultrasound-based diagnostic imaging. By looking at the way these vibrations propagated through the core they derived that part of the inner core has different properties.
Iron crystals in the outer layer of the inner core are aligned according to the north-south axis while iron crystals in the inner part of the inner core are aligned according to the east-west axis suggesting that the inner core could have had ‘a very dramatic change in its deformation regime.’ This information might help us in understanding how our planet has evolved.
Further reading: http://news.illinois.edu/news/15/0209innercore_XiaodongSong.html