It's not ready for primetime quite yet, but Hitachi says it has developed a Blu-ray disc with twice the storage capacity of a BD-50 disc.
Although other companies (including TDK and Panasonic) have developed similar prototypes, each have required a specially developed optical head in the player to read the disc.
By contrast, Hitachi's new four layer disc is believed to require only a firmware update to be compatible with existing drives.
According to a report from PC Pro, the manufacturer is working to stabilize signal quality before considering a commercial roll-out for the new discs.
Like the 51 GB HD DVD disc currently in development by Toshiba, it's expected that if Hitachi's 100 GB Blu-ray discs do hit the market, it will be for data storage use to begin with (and not movies).
PC Pro reports that Hitachi is also developing an eight layer disc that would hold 200GB of data.
Thanks to Alistair for the tip!