Why are all sources of data on the layered structure indirect ?

1. Direct observation is not possible :

The centre of the earth lies at a distance of about 6400 km from the surface of the earth. Man has been able to dig a hole only about 3 km deep for oil and the deepest mine of the world in South Africa is only 6 km deep.

2. High temperature : The temperature in the interior of the earth increases by 1°C for every 30 metres depth. Though this rate is not constant even then the temperature at the centre of the earth is about 2000°C. Because of this high temperature, the direct study of the interior of the earth is not possible.

So the interior of the earth is beyond the reach of direct observations and our knowledge about it is based upon indirect scientific evidences such as earthquake waves and meteorites, earth's gravitation and magnetic field.