I've had CDs degrade over time, in odd ways too... Sometimes the silver film peels away from the edges towards the center, and I've even had a few CDs succumb to something quite strange...

The silver data layer appears to have been "eaten away" by something, between the plastic and the coating protecting the data layer. It leaves fractal like trails and renders the CD unusable.

Both these have happened to CDRs as well as retail CDs. I'm at a loss to explain them, but short of making several copies and checking them periodically, what's the best format for lengevity?

"...but short of making several copies and checking them periodically, what's the best format for lengevity [sic]?"

IMHO, the best format for longevity is hard copy, but it is a PITA to restore anything from hard copy. =-)

The best practice is redundancy at multiple locations with periodic integrity checks. I know it's not the answer you want, but consider the options. You can either have data loss or sometime while you are watching a football game, spend a few commercial breaks checking a few backup disks

For most home users it is viewed as overkill to have a RAIDed data drive in your computer with tape/DVD backup at your house and another tape/DVD backup sent to your Mom's house in another city. Plus a periodic check to make sure the media is still viable.

Saving that, do your backups like voting in Chicago (early and often). Even having 2 backup copies both stored at your house (e.g. one in the office one in the basement) is better than one copy.

Lock up your carbon in gases to preserve old tapes?

Is CO2 sufficiently inert to inhibit mold growth?