What is "_csv" in Python?

In attempting to read the source code for the csv.py file (as a guide to implementing my own writer class in another context) I found that much of the functionality in that file is, in turn, imported from something called _csv:

 from _csv import Error, __version__, writer, reader, register_dialect, \
                  unregister_dialect, get_dialect, list_dialects, \
                  field_size_limit, \
                  QUOTE_MINIMAL, QUOTE_ALL, QUOTE_NONNUMERIC, QUOTE_NONE, \
                  __doc__

I cannot find any file with this name on my system (including searching for files with the Hidden attribute set), although I can do import _csv from the Python shell.

What is this module and is it possible to read it?