Please have a look at the Wikipedia (german) page for this very interesting crypt. It was discovered in the 70s when the subway was built.
It's one of Schrammel's (one of my flickr friends) favorite places in Vienna. So far there doesn't seem to exist a decent picture of the Virgilkapelle. The main reason is that it is usually not allowed to get in there. It lies approximately 12 meters beneath the Stephansplatz. You can see it from the subway station through a completely dirty window. I tried to make the best out of it and I like the result considering the circumstances.