Let's Get Back to the Constitution??

I hear a lot of people saying "We should get back to the Constitution!" And I have to wonder what they actually mean. Do they mean the Constitution we have NOW? Or the Constitution of 1787 or the one with the Bill of Rights in 1791? Or the Constitution leading up to each subsequent amendment? There are a lot of stages to it, and I have to wonder: to which one they are referring.
There are several modifications and augmentations this document has undergone which makes me wonder: just how much of the original document is left?