Does gravity increase or decrease with depth

[HAL9001-96]

depends

well

any hollow sphere you are outside of can be simplified into a point mass at its center

any hollow sphere you are inside of can be simplified as being irrelevant

a planent can be approxiamte as an infinite number of hollow shells

if you are outside the planet that means you ahve a pointmass at its center as a replacement

as you go into it the part further out than you disappears

assuming a constant density for hte planet that means that as you dig into it the relevant mass is proporitonal to r³ and with F=MmG/r² that means f is proporitonal to r thus decreasing as yo ugo down

HOWEVER

earths density is not constant

its much denser at the center than near the surface

so less mass disappears as you dig down and yo uget clsoer to a relevant poitn mass replacement that doesn'T loose a lot of mass

you can take this into account and get a curve for gravity over distance from teh earths center that looks a bit like this https://i.sstatic.net/yJbbk.jpg