Microsoft has documented the file format somewhere...

It’s an international standard (see Office Open XML):

ISO/IEC 29500: Information technology -- Document description and processing languages -- Office Open XML File Formats

available for free download from http://standards.iso.org/ittf/PubliclyAvailableStandards/index.html.

(The documentation of this standard — in 4 parts — currently requires 6,766 pages of PDF plus 1,559 supporting files. :-o)

Cheers,