...Probably not, but I'd LOVE to be wrong.
A set of photos purportedly taken from an exterior "Dark Knight Rises" set in Jaipur, India - which I'm unable to directly link to at this time - show a deep pit with a huge green-screen tarp at its bottom. Typically, this technique is used for making a "real" hole appear filled with something (lava, slime, water or "something else") in post-production CGI. And if you know your Batman, animated series in particular, you know what conclusion people are jumping to.
Do I think that's what it is? No, I doubt it. But would it be awesome? Yes. Yes it would.