Magnetic tape
Declared dead since the 1980s and still the cheapest way to store a petabyte you rarely need to read.
The IBM 726 attached tape to a computer in 1952 and displaced punch cards as the main data medium. Access is strictly sequential — to reach the end you have to wind there — which shaped the whole practice of batch processing, with sorted files and merge runs that assumed data arrived in order.
Its survival comes down to economics and physics. Tape costs a fraction per terabyte compared with disk, draws no power sitting on a shelf, and lasts for decades. It also provides an air gap: a tape in a cabinet cannot be encrypted from a network intrusion, which has made tape backups relevant again in the ransomware era. Modern installations use the LTO standard and robotic libraries.