What's being curved, when mass bends the "spacetime continuum?" Okay, I've read lots of books about Einstein's spacetime being curved by mass and all, but I've never read a really good hypothesis as to what, exactly, is being curved. You can't curved nothing (sounds like a double negative, but it's not, I guess). If there is nothing present, then there's nothing to curve. And, if something is being curved, then it's some kind of entity/phenomenon that being acted upon by the mass in some way. What is that? All of the nice graphics of rubberlike sheets and their grid lines being bent in the locale of a star or whatever are great to look at, and they get the basic idea of mass being responsible for the curvature of spacetime, but those are simply two-dimensional models of something much different that's taking place in three space dimensions and one time dimension. I'm not really expecting a definitive answer, as such, because, if anyone had one, they'd be in Stockholm, standing before the king of Sweden. So, please, if you respond, don't pontificate. I would just like to hear what others might visualize, when they think of "spacetime" in this context.