Here's a torrent of all the images: https://ubuntuone.com/4VxOo5cnInZWQUEplIdb2c
Going by the other comments, I suspect this is missing some. vladoh claims there's even more stuff under the tunnels. Meanwhile, mayanksinghal points out the size of the array holding everything -- it appears north goes up to 14, and south goes down to 25; however, the images in this torrent go only as high as north 11 and south 25. (Seems everyone agrees that the east and west boundaries are 48 and 33.)
https://gist.github.com/3748257
Warning: 6mb worth of files.
Edit: Github repo with all the images and the script, so that people don't need to hammer the xkcd server:
perl -e'print "<style>img{width:2048px;height:2048px}</style><pre>"; print /X$/ ? "\n" : "<img src=$_.png>" for @ARGV' {{13..1}n,{1..19}s}{{33..1}w,{1..48}e,X} > index.html