Jeffrey Andrews-Hanna

It was because there was something fundamental about the structure of the Earth's crust below the Himalaya that people did not know at that time.

Because if you have a mountain range like this, it's got a lot of mass within it and that will deflect the plumb bob towards it.

Yes, that's true.

And if that's the end of the story, then Everest and Pratt would have observed the deflection they expected.

In actuality, though, what we have, if this is the crust of the Earth and this is the Himalaya, below the Himalaya is a thick, thick crustal root extending down into the mantle.

So we now know that the Earth has a low density crust and a higher density mantle, and that this is the arrangement in the Himalaya and in many other mountain ranges as well.

It's similar to what you have with an iceberg.

So yes, with an iceberg, you've got this mass of ice that's jutting up into the air.

This would give you a positive gravity anomaly.

But as we all know, 90% of the iceberg is hidden below the surface of the water.

This ice has a lower density than the water that surrounds it.

Because of that, this would give you a negative gravity anomaly, and in fact, the two happen to cancel out.

And so if you were to try to measure the gravity anomaly from an iceberg, you'd detect a very, very small anomaly, a lot smaller than you'd expect from the 10% that you see.

The same is true of the Himalaya on the Earth and of a lot of other mountain belts.

This is just an amazing discovery made 150 years ago with a weight on a string.

This is the kind of thing we want to be able to do on the moon.

But you can imagine if you could go to smaller scales with better data,

you could detect more subtle things.

So what if within the crust of the Earth, what if there was a dense object?

Something like, say, solidified magma that forms a dense igneous rock.