In Windows, what do you get if you change the extension of an (unprotected) Excel file from .xlsx to .zip?

In Windows, what do you get if you change the extension of an (unprotected) Excel file from .xlsx to .zip?

• A file that MS EXCEL cannot open
• Nothing useful at all
• A corrupt ZIP file
• The original spreadsheet as a set of XML files within the ZIP 

EXPLANATION

Since Office 2007 Microsoft has used Open XML to store non-password protected documents.  These are then combined into a single compressed file using
the  docx, xlsx, pptx, etc. extensions.   Even though the XML files are quite large, they compress very well.