Comment by stephenjudkins

Here's one theory: https://storagemojo.com/2014/04/25/amazons-glacier-secret-bd... and here's the HN thread: https://news.ycombinator.com/item?id=7647571

Facebook uses Blu-ray discs with a robotic storage system for its cold storage: http://www.datacenterdynamics.com/it-networks/robot-digs-thr...