I wonder what they did with them.
Here's something I read very recently (at home, over a bout of rollerblade-related bursitis) that made me think about you and what your thoughts on this might be (I added Sagan at the end just for flourish):
"Fermi pointed out that an intelligent species that developed the sort of technology we have today, and kept on progressing, could be expected eventually to work out a way to travel from one star system to another; they would also leave traces that would be detectable from earth. Even if interstellar travel proved to be slow and difficult, a species that developed starflight technology could colonize the entire galaxy in a few tens of millions of years - in other words, in a tiny fraction of the time the galaxy has been around. Given 400 billion chances to evolve a species capable of inventing interstellar travel, and 13 billion years to roll the dice, the chances are dizzyingly high that if it's possible at all, at least one species would have managed the trick long before we came around, and it's not much less probable that dozens or hundreds of species could have done it. If that's the case, Fermi pointed out, where are they? And why haven't we seen the least trace of their presence anywhere in the night sky?
...On another level, though, Fermi's Paradox can be restated in another and far more threatening way. The logic of the paradox depends on the assumption that unlimited technological progress is possible, and it can be turned without too much difficulty into a logical refutation of the assumption. If unlimited technological progress is possible, then there should be clear evidence of technologically advanced species in the cosmos; there is no such evidence; therefore unlimited technological progress is impossible. Crashingly unpopular though this latter idea may be, I suggest that it is correct - and a close examination of the issues involved casts a useful light on the present crisis of industrial civilization...
The difference between going to the moon and going to the stars... isn't simply a difference in scale. It's a difference in kind. It takes literally unimaginable amounts of energy either to accelerate a spacecraft to the relativistic speeds needed to make an interstellar trip in less than a geological time scale, or to keep a manned (or alienned) spacecraft viable for the long trip through deep space. The Saturn V rocket that put Apollo 11 on the moon, the most powerful spacecraft to date, doesn't even begin to approach the first baby steps toward interstellar travel. This deserves attention, because the most powerful and technologically advanced nation on Earth, riding the crest of one of the greatest economic booms in history and fueling that boom by burning through a half billion years' worth of fossil fuels at an absurdly extravagant pace, had to divert a noticeable fraction of its total resources to the task of getting a handful of spacecraft across what, in galactic terms, is a whisker-thin gap between neighboring worlds...
...Finally, the suggestions made here raise the possibility that for more than a century and a half now, our own civilization has been pursuing a misguided image of what an advanced technology looks like. Since the late 19th century, when early science fiction writers such as Jules Verne began to popularize the concept, 'advanced technology' and 'extravagant use of energy' have been for all practical purposes synonyms, and today Star Trek fantasies tend to dominate any discussion of what a mature technological society might resemble. If access to concentrated energy sources inevitably peaks and declines in the course of a technological society's history, though, a truly mature technology may turn out to be something very different from our current expectations."
-- John Michael Greer
"Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves... There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known. [and may ever will know]"
-- Carl Sagan