[goofkid (OP)]

What would happen if we kept digging deeper and deeper through the earth's surface?? At what point could we dig no more and why??

[another_someone]

The point when things would start to get a bit hot.  Ultimately, it will get hot enough to be melting the rock around you, so you would anyway not be able to dig liquid rock (liquid rock under high pressure).

[goofkid (OP)]

so....you can't make a hole through the earth??

[eric l]

The deepest hole dug so far is about 0.1 % of the earth's diameter deep.  New technologies will probably allow deeper holes, but there remains a lot of digging to do.

• http://www.damninteresting.com/?p=567
• http://www.madsci.org/posts/archives/mar97/855593397.Es.r.html

[another_someone]

so....you can't make a hole through the earth??

Using any present, or presently conceivable, technology; the answer is no.

But then this question is slightly different to the one you asked above.  You asked about digging a hole, not about making a hole. 

It is conceivable that one could burn a hole through the Earth (this was the premiss to the China Syndrome - although the reality is that it would be far harder, and require a lot more energy to do, than implied by the China Syndrome).   The process of doing so would, if possible, probably cause significant damage to the Earth (given the enormous amount of energy you would need to achieve it).

First question would be how you stop the hole from filling in behind you (given you are moving through molten rock at very high pressure)?  Making a hole in liquid is like trying to make a hole in the sea - you might be able to do it, but you would not be digging the hole into the sea.

Second problem is, if you have a hole from deep down in the Earth right up to the surface, then this will allow a lot of heat to escape.  All of this heat, once it reaches the surface, if likely to do some damage there; and in having that heat escape from lower down, you would be effecting the convection currents within the Earth itself.

You are also going to have to displace a great deal of rock - where do you put it all - you will be building a fairly big mountain (in fact, I doubt there is any mountain on Earth that contains as much rock as how much you will be digging up from beneath the ground).

It is conceivable that some high energy extra-terrestrial object could slam into the Earth, and go down to the core - but in the process, it will probably shatter the Earth apart.