Which layer of the Earth are we standing on?

The crust, and it is thinner than almost anyone pictures. Under the oceans it averages about six kilometres; under the continents around thirty-five. Set against a planet 6,371 kilometres in radius, that is proportionally thinner than the skin on an apple. Everything that has ever lived has lived on it.

Nobody has been through it. The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, reached just over twelve kilometres after two decades and stopped because the rock at 180 degrees behaved more like plastic than stone and kept closing the hole. That is about a third of the way through continental crust and roughly a five-hundredth of the way to the centre.

Everything known about the deeper layers is therefore indirect, and mostly comes from earthquakes. Seismic waves change speed and direction as they pass through materials of different density, and some types cannot travel through liquid at all - which is how it was established in 1936, by the Danish seismologist Inge Lehmann, that the Earth has a solid inner core inside a liquid outer one. She worked it out from the arrival times of waves that should not have been there.