I recently downloaded a zip archive of freely-distributed MP3 files. But when I double-clicked it to decompress it, all I got was a .cpgz file (an archive file in the CPIO format). Double-clicking that file just created the zip file again, and thus I was stuck in an endless loop
Searching on Google, I saw that plenty of other people had come across the same problem. Some of them suggested some fixes, but none of those fixes worked for me. Some online posts suggested that I download the file again, because the first file might have become corrupted. But as it was a very large file, I really didn't want to try that.
The solution for me was to use the free The Unarchiver, which turns out to be a Swiss army knife for decompressing many obscure types of archives. Opening the file with The Unarchiver decompressed it correctly.
In some cases, The Unarchiver may not work. If this is the case--or if you simply prefer using Terminal--you can try using the unzip command. Just launch Terminal, type unzip, then a space, then drag your archive into the Terminal window and press Return. That will decompress many files that are resistant to other techniques.