Members of an Italian herritage association claim that tunneling 50 meters below the streets of Rome would put the Roman Colosseum at risk of collapsing. Here is an excerpt from the article I found:
"The Colosseum is at risk," heritage association Italia Nostra said in a statement. "Nobody at the moment can guarantee that the Colosseum’s foundations will not be affected by the deep excavation of land just dozens of meters away from the monument," it added.
This is certainly an interesting problem, but as a geotechnical engineer, I’m not so sure that it would be impossible. Certainly any tunneling or excavation would require monitoring and perhaps more active measures to ensure that nothing happens to the Colosseum, but it seems like this is more of a political statement than a technical one. [Source: IOL.co.za. Image: Wikipedia, Jean-Pol GRANDMONT]