A complete Mathematica installation, has c, and some others, code accessible by users. It is found, among other places, in
/usr/local/Wolfram/Mathematica/8.0/SystemFiles/Links/Python
Therefore, one can interact with Mathematica at the code level. Obviously one has to know those languages.
This screenshot is from the c code mathlink.c, in the Python directory. Also there are a few more .py programs there.