What storage format is reasonably likely to be long lasting?

When I was younger, we were told that optical media would last 50 years or more. (That may be true for stamped optical media, but) it was a lie for burned optical media.

Your CD-Rs and DVD-Rs are probably already failing! Go archive them.

Magnetic tape fails. Spinning disks suffer head crashes.

What's the lifespan of a piece of flash media that is written to once?

Is paper really the only archival format?

(I say this as someone who uses floppy disks more than once a month, who has a library of audio cassettes and VHS tapes.

Well made magnetic tape fails fairly slowly, if it's stored well, but it still fails.

Shellac and Vinyl records have, thus far, proven to be our most stable music archival format.

What's the equivalent for data?)

All the professional archives that I'm aware of use some variant of a personal cloud storage system with redundant backups and constant human maintenance.

Most folks in this thread are suggesting something called M-Disc.

I guess I can try some M-Discs and see if they're still readable in four decades, but anything significant also needs redundant backups and human maintenance.

I can't find m-disc DVDs for sale.

I can find m-disc blu-ray discs for sale.

For archival, blu-ray is obviously preferable. For redundancy and cost, as well as maximal reader compatibility, I'd like the DVD option as well.

Either way, these discs are exponsive. Multiple dollars each, sometimes as high as $10/disc. (Granted, that's for 100 GB discs, so it's only $1/10 GB.)

The 25 GB discs seems to hover in the $3-5/disc ballpark. (Or roughly $1/5 - 8GB)

I dunno what to do with this data for many reasons, not the least of which is that I am still pretty skeptical about m-disc as a format, and I'm having a hard time finding any third party testing to support their longevity claims.

Yesterday I asked "Which storage format is reasonably long lived" and I got a lot of "clay tablet" answers.

I went to bed.

I woke up this morning with even more "Clay Tablet" answers, and a few folks being extra condescending and smug.

So let me revisit this thread some more, so that we can figure out if there's something to be gained here.

I'm asking about digital storage formats that are reasonably long lived.

The word "reasonably" was in the original post, and digital was pretty heavily implied, but not explicitly stated.

So let's re-state the question in clearer terms:

What digital storage format is most likely to last 50 years in cold storage?

Clay tablets aren't a digital storage format.

Paper could be made to store digital material, especially using some kind of optical encoding or punch tape, but the information density is pretty low and it would require specialized equipment to read.

How long does Flash last in cold storage? Would an SD card in a safe deposit box be readable in 20 years (assuming you could find the right adapter?)