Successful archiving of data requires a storage medium capable of persisting for times measured in centuries, and providing an absolute trust in supportable and permanent hardware. While all media decay, data on magnetic media, solid-state drives and conventional optical disks must be cyclically refreshed over relatively short time frames, requiring energy and labor resources. We propose a "Write-Once, Read Forever" (WORF) module specifically engineered for long-term preservation of digital data. It uses a novel high-density data storage medium based on silver halide, which has been demonstrated to last for more than a century under normal ambient environmental conditions. Once data is written to WORF, energy is needed only for reading—no periodic refresh is necessary, and data is both immutable and truly permanent. Human readable text and images are embedded in the WORF module adjacent to the digital data. This text and imagery contains metainformation about the media's content, and instructions for decoding for future generations. WORF digital data is stored as microscopic, metallic interference gratings (representing wavelengths or "colors") embedded in a modern, super-resolution, dye-free, photosensitive emulsion. Wavelengths encode multiple states per data region; current spectroscopic technology makes 400 states per 2 micron diameter data region feasible. Multi-state data architecture within each domain enhances data integrity, error-checking, and accelerates writing and reading for the entire media module.