Sony has developed a higher-capacity version of their Optical Disc Archive (ODA), which now allows up to 3.3 TB of archival storage with the promise of 100-year retention.
Sony ODS-D280U (Image credit: Sony via Computer Base)
Of course the viability of such a system in the next century is unknown, and a working cartridge (which is similar to the multi-CD systems found in cars a few years ago) would be needed to access the data. The idea is certainly interesting considering the potential for failure with traditional hard drives, though hard drives are relatively inexpensive and offer more utility, unlike the write-once Sony ODA cartridges.
Cartridge exploded view (Image credit: Sony via Computer Base)
For those seeking pure read-only archival storage, the higher capacity of the second-generation Sony ODA at least brings it closer to parity with current hard drive storage.