|Stainless Steel Fermentation Tanks at a popular winery in Northern California - Nikon D80|
Cold storage in the context of Internet jargon refers to the storage of cryptographic keys (private key) on some type of physical medium that is not connected to the Internet and is not susceptible to external electric fields. A Faraday cage placed below the surface of the ground could be a choice. Since cryptographic keys can be copied, the keys are stored at multiple geographic locations for redundancy in case of natural disaster.
For an explanation of private keys, key parirs, and symmetric vs asymmetric systems, please see my paper on Public Key Cryptography.