Last Train to Yesterday

Imagine a world sometime in the not-too-distant future where the secret to time travel has been unearthed. I say unearthed because that secret had actually been found and tested over a century ago. The first discovery had been greeted with wild jubilation by scholars and the University. The impossible had been achieved! Time explorers approached their work with unbridled enthusiasm. They returned, terrorized by the prospects of apocalyptic doom. What if their actions caused a cascade...
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Imagine a world sometime in the not-too-distant future where the secret to time travel has been unearthed. I say unearthed because that secret had actually been found and tested over a century ago. The first discovery had been greeted with wild jubilation by scholars and the University. The impossible had been achieved! Time explorers approached their work with unbridled enthusiasm. They returned, terrorized by the prospects of apocalyptic doom. What if their actions caused a cascade of uncontrollable events that changed the present reality? What would it take to cause the extinction of mankind? Could killing a mosquito, or not doing so, create a pandemic of epic proportions. No one had thought these possibilities through.
Just over one hundred years later, two ambitious young college students open an old trunk in their great-grandfather's attic. They are looking for clothing to wear to a Halloween costume frat party. In the bottom of the trunk, the boys find a large file of papers wrapped tightly in a red ribbon. Large red letters across the folder spell out the word "SECRET". Underneath the word, in smaller letters, it says, "To Be Destroyed!"
The boys' curiosity is engaged. Certainly, these documents are no longer classified. This file must be 100 years old. There could be no harm in taking a look-see. Right?