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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):
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?