Last week I was in the Köbanya mine in Budapest. It was in the Middle ages a limestone mine, and then was a big big big cellar system. You still see that it is a cellar system and some parts seems to be still in use as cellars for wines.
There are 4 flooded areas, we had only time to dive 2 sites. With your own car you drive into the callar system to the divesites. It is really huge, a complete maze.
The entrance is just a stairs down to the water:
Under water you don't see rails, but stairs and stairs. And some tunnels. The water is clear and around 13 degrees C year round. It is not a difficult dive, just follow the only line that is in there, it is not big, but it is a nice place to take pictures. If I have the chance, then I go back for some more days diving here.