Sometimes it happens and you find yourself facing a huge amount of legacy code. It is developed by persons who are no longer available for one reason or another. There is practically no useful technical documentation and the business cases have mutated something totally different during the years, bugfixes and change requests.

But now you have the code and the deadline approaches. You should implement some changes to this application. Unfortunately there are no test setup to ensure your changes wont break anything. Actually that is not a problem yet because you do not know how to do the first cut.

What to do...?

Take a notepad and a pencil - maybe several colors. Start exploring the code, run it through debugger, make some experimental changes. Mark everything down as a Mind map . After a while you will start to figure out how things are working and what their relations in the system are. There is software for Mind maps but I prefer pencil and paper.

Are there any other tools are techniques to explore and learn unknown code?

Further reading:
The Art of Maintenance Programming
The Noble Art of Maintenance Programming