Increase in density as you get closer to the core.

On top of which gravity is going to lessen as you get to the center of mass, so even a little resistance will eventually become extremely significant...

Only 100 km down, the gravitational acceleration would not be reduced by very much. Assuming the Earth is spherical and has uniform density, the probe would only be below ~3% of the mass and would be less than 2% closer in distance. So the acceleration felt by the probe would only vary by a few percent.

At 100km, the gravitational acceleration is actually higher than at the surface.

http://www.wolframalpha.com/input/?i=gravitational+accelerat...

In fact, it increases until 3500km below the surface (2800km radius).

http://en.wikipedia.org/wiki/Gravity_of_Earth#mediaviewer/Fi...