The cold, crisp April morning air was shattered by the blinding flash of a 10 megaton nuclear detonation. Shortly thereafter, other explosions occurred. With each flash, one of the world's great cities was ground into billions of tiny radioactive particles. The bombs continued to explode until there were no more human fingers left to push launch buttons.
There then followed a period of time which, by human measuring, amounted to roughly 5000 years.
The alien ship surveyed the ruined landscape of the Earth from a high orbit. The Science Officer reported to the ship's Commander.
"It would appear, sir, that they destroyed themselves, with the use of nuclear explosions."
"Why?" asked the Commander.
"We don't know, sir. The computer's best guess is that they took their own lives in order to prevent unleashing some dangerous force upon the rest of the galaxy."
"Radiation levels are very high on the surface of the planet," continued the Science Officer, "but with proper protective gear, archaeological teams could explore some of the ruins."
"Carry on, then," said the Commander.
After about a week (measured by human time), the archaeological team gave their report.
"We found very little intact. Mostly just bits and pieces, now junk. With more time, we could learn more. However, we did find these." The Chief Archaeologist indicated two large, torpedo shaped containers. "These two vessels contain a large number of artifacts that tell us a certain amount about the civilization that once lived here. It seems that at some point in their history, they assembled these two canisters in order to provide future generations with a record of their accomplishments. The two containers were clearly designed to endure a long passage of time, though it's fairly surprising that they survived this long, since they were located quite close to a major nuclear blast crater. The contents seem to have been selected to be representative of the people who left them. Our current translation efforts indicate that these vessels were called 'Time Capsules' and were deposited at something called the 'New York World's Fair.' In any event, we've decided to take them back to the Home World for further analysis."
Several human decades later, the contents of the Time Capsules found their way into a small glass case in a dusty back corner of the Museum of Alien Cultures. Most visitors to the museum did not notice the exhibit at all, and those who did found it only mildly amusing.