10,000 years in the future
Via Matt Webb, 99% Invisible considers how to communicate details of where radioactive material is buried to people living 10,000 years in the future.
Bastide and Fabbri came to the conclusion that the most durable thing that humanity has ever made is culture: religion, folklore, belief systems. They may morph over time, but an essential message can get pulled through over millennia. They proposed that we genetically engineer a species of cat that changes color in the presence of radiation, which would be released into the wild to serve as living Geiger counters. Then, we would create folklore and write songs and tell stories about these “ray cats,” the moral being that when you see these cats change colors, run far, far away.
Maybe they should look at geomythology, as covered by Nautilus:
The late geologist Dorothy Vitaliano coined the term geomythology in the 1960s to describe oral lore that explains peculiar landforms or references geological cataclysms—earthquakes, fires, floods, volcanic eruptions, diverted waterways, or the sudden emergence or disappearance of islands. Like most geologists, I once dismissed these accounts as imaginative fantasies. Embellished with supernatural details and shrouded in the language of myth, they rung no truer than science-fiction yarns about Martian colonies and cyborg races. There is some evidence, however, that many geomyths are in fact grounded in events that actually happened.
The story of the great flood is one of the oldest and most widely told. A version of this legend appears in so many cultures that some pseudo-scientific theorists have invoked its ubiquity as evidence of a global flood. But while flood myths share common elements—catastrophic inundation; a harrowing escape, usually by boat—the nature of the deluge varies from region to region. And the differences are telling. Often, the features of a fabled flood bear a striking resemblance to local geological processes, suggesting that many myths record real catastrophes witnessed in antiquity.