Answer: Constant

In time travel stories, you will often find something that exists in each time period the traveler visits. This something, typically a geographic feature, man-made landmark, or even a small object the protagonist stumbles across, serves to anchor the story and establish that the traveler is in the same place.

If the time traveler, for example, carves his name into a tree when he is a boy, builds a time machine as a young scientist, and then travels far into the future, then the audience might be shown the same tree as a very old and mature tree with his name still carved in it to establish that he is in the future. The tree, then, serves as the thing that remains constant and stable between the two timelines.

In the iconic time traveling film series Back to the Future, for example, the town’s clock tower appears under construction in 1885, completed and functional in 1955, broken down in 1985, transformed into a casino-hotel in the alternate 1985, and upgraded to a futuristic piece of art in 2015.

While that’s certainly something a sharp eyed viewer would note, there are few time traveling constants that can hold a candle to the impact of the constant found in 1968’s Planet of the Apes. At the end of the film, the protagonist finds a broken down Statue of Liberty buried in the sands of an ocean cliff—seen here—and reveals to himself and the audience that the world he has tumbled into is not some distant universe or parallel dimension, but Earth in the future, long after mankind has fallen.