It is in fact possible to keep and offline mirror of Wikipeda for yourself. But just the text of it obviously. If you tried to download ALL of every Wikipedia article, including sounds images and videos, it would probably be a couple terabytes in size.
Now, I can't for the life of me though figure out how to do it in Linux. I can find page after page of how to do it in Windows, but nothing for Linux. Help please?
http://www.quickonlinetips.com/archives/2012/08/download-full-wikipedia-content-offline/

https://en.wikipedia.org/wiki/Wikipedia:Database_download

This seems to imply that you can download an archive and extract it, but media files have to be downloaded separately.

>>2
wikipedia is just the text; images are from wikimedia commons (which also supplies images to many other wikis)
so yeah, no images

also, i have the 2012/10 version and it's a bit less than 30 GB
woth downloading, if you ask me.

>>1
Goodness me, I never knew such a thing was possible! Thank you for enlightening me. I have just downloaded the most recent version.