Consider Herodot's account (he lived in the 5th Century BC) about the circumnavigation of the African continent by a Phoenician expedition under Pharao Necho II, about 150 years earlier. The fleet set out in the Red Sea and returned three years later through the Strait of Gibraltar.
Herodot, who clearly had access to the original records, which have since been lost, like so much of Classical material, says that as they rounded the continent the sailors had the Sun to their right, but he, Herodot, cannot believe this because everyone knows that at noon the Sun is in the South. A more complete account of the matter is here: http://www.livius.org/he-hg/herodotus/hist01.htm
Knowledge is fragile, much more fragile than I like.
Also, if Earth were to take a direct hit from a large coronal mass ejection like the Carrington event of 1859, there is no telling how much data might be lost. "Forever" is never a good assumption to make about anything.
It seems like we've made the tradeoff towards increased convenience of access vs. reliability, not the other way around. Hopefully new tech is developed that reverses this at some point, without relying on higher-level systems built on top of unreliable media (e.g. Amazon Glacier, Archive.org, etc.)
[1] http://sportsvideo.org/main/files/2010/08/video-tape-white-p...
The Empire itself doesn't exist anymore sure, but I don't think the GP was talking about political structures.
Ancient Romans' culture never disappeared, we can still easily understand their language (some people can speak it, I learned it in school, and there's even a latin Wikipedia! and we still use it for creating new words in our modern languages) and their culture. Their culture has evolved into our modern European culture (mixed with others, to some degree), our modern democratic republican political systems are a mix of two systems both older than the Roman Empire, and there are no mysteries about aspects of their culture or infrastructure.
If the Romans had written a warning on a building in plain Latin about how entering it would unleash the wrath of the gods, we would have absolutely no problem to understand it.
We would still enter it, though, but not out of ignorance, we would enter it because we know we can today handle anything the Romans might have feared.
All in all, I think Romans are a pretty good example of how a powerful culture can stay understandable for several millenia.
Today we can, the privileged few in our current golden age of technology and widespread literacy. Take the non-privileged masses in the US and elsewhere, they wouldn't know what to do with Latin script written on a building, even now. They wouldn't know how to translate it on the web, they wouldn't even know it was a warning.
It's not about knowing the meaning of the site. It's about knowing the meaning of the site with little or no loss of critical information.
This may not seem like a big deal, but it actually is. Technology didn't improve that much between ten thousand years ago and 150 years ago. This is why we're able to examine ancient sites and kinda sorta make something out. But the rate of technological progress has been increasing, and this acceleration has no end in sight so far. What that means is that ten thousand years from now, our descendants may have no way of relating to the way we do things now. That's why we need to think really, really hard about how to make the information survive in a way that is still digestable to a hyper-advanced civilization.
I don't think hyper-advanced civilization would have a problem with this - they would either have not lost the knowledge or be able to figure out the danger themselves by using their hyper-advanced sensors and stuff.
The problem is that if our civilization collapses and the Earth goes back to 2000 B.C., those folks will need some warning not to mess with a deadly force that is beyond their comprehention before they reach sufficient level of advancement to know what they're dealing with.
Consider how we figure out what sites meant 1000+ years ago... We dig them up.
Doing that to a waste disposal site would be phenomenally bad.
Even before digital storage and the like, is there really any relevant information that has been lost in the past 150 years? And as to the whole "language barrier" thing, we figured out hieroglyphs with a much smaller corpus.
Hell, we know about the meaning of loads of sites from over a thousand years ago.
I feel like we'll have ways of knowing what the site contains forever.