According to Opening MPK file without ArcGIS Desktop?, .mpk files are just .zip compressed and I should be able to unzip them as normal. I've tried from the terminal on macOS (10.11.6):
unzip somefile.mpk
for multiple different .mpk files I always get this error:
Archive: somefile.mpk
End-of-central-directory signature not found. Either this file is not
a zipfile, or it constitutes one disk of a multi-part archive. In the
latter case the central directory and zipfile comment will be found on
the last disk(s) of this archive.
unzip: cannot find zipfile directory in one of somefile.mpk or
somefile.mpk.zip, and cannot find somefile.mpk.ZIP, period.
I get the same error if I change the file extension from .mpk to .zip. These files are known to work fine on Windows, when uncompressed with ArcGIS tools. They can also be unzipped with 7-zip on Windows.
Any idea what's going on here?