I've been speaking to a number of very large IT organizations many which have large IBM mainframes, and a host of other systems dating back to the 1950's. The common thread is that very few companies have any documentation of systems that have spanned over 50 years of use.
The knowledge of these systems is contained in the heads of developers that are nearing retirement age. As companies lose key people, they lose tremendous knowledge of existing systems.
In a world where we have tools to automate documentation of systems few companies have invested in these tools. I am astounded that companies put themselves at risk over a tiny sum of money when compared to the total IT budget.