A little pushed for time this evening, so please can I send you elsewhere (and request that you come back here again tomorrow)?
And the elsewhere is this CNN article on the hidden thermal caves beneath Budapest.
Some really interesting information:
Molnár János is still alive and growing. Water rich in hydrogen sulfide and carbon dioxide continues to percolate through the rock, creating a mildly acidic cocktail that eats at the walls. The result is a Swiss cheese labyrinth of chambers and passages.
And some stunning images:

I’ve been to Budapest a few times, and I had no idea about any of this.
I probably should have asked more questions.