tl;dr: M-DISC is a write-once optical format (DVD, BD-R, BDXL up to 100GB) introduced in 2009, originally using an inert glassy carbon data layer claimed to last up to 1,000 years, and readable in most standard drives. The original developer Millenniata went bankrupt in 2016, and in 2022 Verbatim controversially reformulated its Blu-ray M-DISCs to use a metal ablative layer (MABL) similar to their standard discs, drawing accusations of deceptive marketing despite Verbatim's claims of equivalent longevity. NIST currently lists M-DISC as an acceptable archival format rated for 100+ years.