One or more copies of the original system (this may require duplicate hardware and software systems, plus the appropriate backups) preserved in a static state.
Pros
Maintenance costs are low if data is not needed on a regular basis (mainly storage).
Con
Cost to access and use data is high (original staff or documentation has be in place to be able to get to data).
If original systems are destroyed or damaged, the data may not be recoverable or may be very expensive to recover.
This option might make sense when you are converting from a legacy system into a new one and want to hold onto the old system in case you need data from it.