Comments from feedback form - "So it may be shrinking. Its ma..."

Referring to your quote "that if you dig into the moon (or any other object) you don't experience an increased gravity because the material on top of you is now pulling you in the opposite direction". Given that scenario, the mass at the center of the earth has all of the mass of the earth outside of it and should therefore be pulled "in the opposite direction". This implies that the earth should be hollow. Isn't this the kind of thinking that led to the "hollow earth" hypothesis. In fact, you can still become a member of the Hollow Earth Society. However, it is currently understood that the density of a massive object is highest at the center. Gravity is a function of both mass and density acting from the center of the mass "as a whole".

No, it doesn't imply that the Earth is hollow. What it implies is that if you dug a tunnel straight through the entire Earth (and used... I dunno... a forcefield to keep it open), at the very centre of the Earth you would be weightless, because all of the Earth's mass would be tugging on you equally in every direction.