You could visit a mining site and go down haha. How flat do the believers think the world is? Is it supposed to be 2 dimensional or are they saying its a few miles wide or what?
I actually never heard about how they think about the depth of earth.
@Batman: Can you dig "through" earth and fall off?
If you go up to the top of a building - don't make it a tall building; 3 stories high is enough - and you jump off, do you fall up or down? You fall down, of course.
The point? If you dug through the earth to the other side, and you fell out of the hole you dug through, can you call that falling up?

If you could actually dig a whole through a (real world) earth and jump through it without getting molten you would eventually move from falling down to falling up, because the center of gravity is in the center. On a flat plane that would be different, hence my question toward the flat earth "truther".