What do you do with undocumented legacy code?

I have A LOT of this to deal with right now. There are whole libraries used across multiple webapps and services that are completely undocumented. No javadocs, no comments in the source code, no flow diagrams, no written explanations, nothing. Some of these libraries don't even have source code attached to them, and of course there are no information about them whatsoever on the internet, so they must have been developed internally, or even worse, borrowed from mysterious sources. Needless to say that the original devs are long disappeared, even before I joined the team. Now what would you do about it? Should we invest time in analyzing it and produce some documentation? Should we rewrite the whole thing? It is worth noting that the libraries appear to work correctly, so you know, don't fix what ain't broken, but it still is a maintenance hell... Inputs will be appreciated