Ok. I think I've got it.

Imagine you're at the centre of the earth.

Imagine a column of iron, molten rock, solid rock etc above you in any direction, say towards the north pole.

Now. You are weightless.

The layer of iron immediately above you in the column is nearly weightless, but not quite. Because there is more mass 'below' that layer (towards the centre and then on to the surface opposite) than above, what nett weight that layer has is vectored by gravity towards the centre. Same for the molten rock halfway up the column, which we could say has a nett 'half-weight' (due to half gravity) also vectored towards the centre. And the crust at the surface has full gravity, full weight, also vectored towards the centre.

The gross effect on you at the centre is to ADD the full weight of the top layers, the half weight of the middle layers, and the not-very-much weight of the bottom layers... add it ALL up and it is ALL pushing on you towards the centre. That's heavy. And there is a similar column in every direction.

Thats gotta hurt!

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein