Illustration of a cross-sectioned globe showing the Earth's internal structure. The external layer of the globe shows the Earth's surface topography and atmosphere, including land and water. This surface layer of the Earth extends downwards for around 35 kilometres as the rocky crust. The mantle is a viscous layer of rocks under high pressures and temperatures, extending downwards to a depth of around 2890 kilometres. The outer core is a liquid layer of iron and nickel, around 2260 kilometres thick. The inner core is a solid sphere of iron-nickel alloy, with a radius of 1220 kilometres. Danish seismologist Inge Lehmann (1888-1933) made the discovery that the inner core of the Earth was solid using data from seismological stations.