What is inside the Earth.

Step 1 of 5

The S-wave shadow

The deepest hole ever drilled, on the Kola Peninsula in Russia, reached about $12$⁠ kilometres: less than two thousandths of the way to the centre. Everything known about the depths comes from seismic waves. Every large quake lights up the inside of the Earth, and seismometers all over the world record what got through and what did not.

Wave type is locked on S waves, and the Seismometer at $120^\circ$⁠ from the quake. Time since the quake will run to the end.

Step 2 of 5

A liquid core

The paths in the Scene show what happens to S waves that go deep: they reach a boundary at a depth of about 2900 km, and stop there.

Step 3 of 5

The edge of the shadow

Wave type is locked. The waves do not travel in straight lines: their speed grows with depth, so they refract all the time, like light, and their paths curve back towards the surface.

Step 4 of 5

The P-wave shadow

Now Wave type is P waves. They go down into the core too, but they do not stop there.

Step 5 of 5

The inner core

Look at the seismometers inside the P-wave shadow: some of them light up all the same, faintly. The layers the waves revealed, from the outside in: a thin crust, a mantle of solid rock down to about 2900 km, an outer core of liquid iron and nickel, and a solid inner core.