They speculated that parrots and other birds like it would still retain some human speech, and that for about a hundred and fifty years or so human voices would still be heard by parrots teaching each other the words they learned, even if only words and small phrases.
In one episode, it's speculated that the last remaining artifact of humankind could be the plaque left on the surface of the moon by the crew of Apollo 11. Thanks to the lack of atmosphere, the words "We came in peace for all mankind" could remain for millions of years.
The fact that our monuments to the world will still be standing at least a few centuries after we're gone.
Best estimates? The Hoover Dam will last for at least 10,000 years, while Mt. Rushmore could still be recognizable after 200,000. The Great Wall of China (which "ages likes mountains") will still be around for eons. And the Great Pyramids at Giza, swallowed up by the sand, might last for millions of years. Some of these will be around long enough to be seen by our successors (whoever that may be).
Washington DC's Iwo Jiwa Monument is fondly described as becoming a defiantly intact memorial of American heroism...long after the rest of America has long been forgotten.
The US alone is shown to have quite a number of potential relics that would endure. The Cheyenne Mountain Complex would still be recognizable thousands of years hence while what's left DC itself is likely to become fossilized, with the words Laus Deo etched on top of the Washington Monument being the Americans' final words to the future. The Liberty Bell, even after it splits in half, remains clearly recognizable even as its surrounded by a forest.
Although the Eiffel Tower and the Mona Lisa will have been long gone, two of France's other cultural icons, the Venus de Milo and the cathedral of Notre Dame, will both survive even after 2,000 years.
The Statue of Liberty's arm and torch, with the ballistic trajectory achieved from breaking off the statue, may leave behind an impression in the seafloor that could be fossilized like a footprint.
The Cassini spacecraft could land on Enceladus, thus giving life to another celestial body via microbes. If there's one impact we're leaving, it's giving life to Enceladus.
The proposed KEO Satellite is described as being a Hope Spot for mankind, being one of the most lasting "time capsules" compared to all the others.
Settlements in the world's extremes like Pyramiden are likely to ultimately be the last standing towns on Earth, as the climate would put the forces of decay and erosion into very slow motion.
Life coming back to regions that had previously been left barren by human activity, like the eventual rebounding of the oceans or of parts of the Southwest where fresh water is presently being diverted away for use in agriculture and cities.
The existence of the human geological layer. The remains of our civilization, from refined materials to plastics, will remain as a unique strata within the Earth's crust. Buried deep down could be artifacts like toothbrushes or old toys, reminders of the lives we lived kept preserved for millions of years.