WORM Storage: 

The characteristics of the "Write Once-Read Many" (WORM)
technologies are similar to those of CD-ROM.  WORM devices are
optically based with a storage capacity from 200 to more than
1000 megabytes.  The WORM disk is a plastic encapsulated,
metallic finished, polycarbonate disk contained in a removable
cartridge.  The disk is designed to be written on by the end user
with a laser device that etches digital data into pits on surface
of the disk.  The data field, once written to disk, is an
unalterable, permanent archive.  New data can be written to the
disk until either file limits are reached or storage volume is
consumed.  Since data cannot be overwritten, a data audit trail,
or backup archive, is always present to preserve the progression
of project or program development.  The anticipated design life
of the disks is 10 years, as opposed to 3 years for magnetic
media, and the data is not susceptible to magnetic fields
(Kalstrom, 1988).

<cfile61>   Optical Disk Storage