5,000 kilometres below the surface we live on, the core of the Earth spins independently in the liquid metal outer core.Presumably, the Earth's core and mantle had always rotated in the same sense. Now the core has slowed down and is starting to rotate in the opposite direction.According to a Nature Geoscience's study, this could shorten days and influence climate and sea level.The authors of the research are Yi Yang and Xiaodong Song of Peking University, China.Song was analyzing waves from earthquakes in the South Sandwich Islands when he discovered that the Earth's core had slowed down in 2009.In the mid-1970s, a similar standstill had occurred."We infer that there is an oscillation cycle that lasts about seven decades. This suggests that there is a resonance connecting all the layers of the Earth that occurs with this cadence", Xiaodong Song of Peking University, China.The geologist states that the rotation of the inner core can alter its gravitational field, which can influence the planet's sea level and global temperature.Researchers at the University of Southern California believe that it is still too early to accept the conclusions of Song's study as valid."It is possible that in 5 to 10 years, with more data and better simulations, we may know if, as it seems, the Earth's core follows these cycles", John Vidale, seismologist at the University of Southern California.