Isofs transparent compression
From Just Solve the File Format Problem
Mkisofs is a utility for generating ISO 9660 image files, and related formats. Some old versions of it support a "transparent compression" feature, implemented as a Rock Ridge extension with signature "ZZ". It appears to be based on gzip format.
History
Transparent compression was new in mkisofs 1.1 (a.k.a. 1.01) (1994). Around 2001, mkisofs was incorporated into the cdrtools software. The feature was present in cdrtools 1.10 (2001). It was removed and replaced with zisofs support in cdrtools 2.0 (2002).
