19 July 2021 1:18 PM (fiction)
Humans had been finding them everywhere: carved slabs of granite, embossed gold sheets, anything that might be read without tools and be durable.
Inside mines, under coal seams, all over the planet in any area with water and good soil or long-lived resources. Some had pictures with symbols underneath them, like a primer for children. Some were glossaries from one unknown language to another. Some had strange diagrams.
Each one ended with the same string of symbols. The humans thought it was a prayer or magic formula. Many took the engravings and set them up in town centers to bring luck, others thought it a bad idea to disturb them and left them be.
Abundant natural nuclear reactors had inspired the invention of the nuclear powered steam engine almost as soon as there was a steam engine, leaving the easily accessible seams of coal largely undisturbed. When electrification came, coal and natural gas barely figured.
As science progressed, something odd was found: the isotopes in the most easily accessible coal matched atmospheric carbon dioxide more closely than they did deeper coal deposits. Also, the natural reactors seemed not as natural as they had at first.
Eventually, the humans learned to translate all the writing they'd found and almost all of it was instructions on how to recognize and a primer on all the technology needed to decode the much denser data storage media that had been preserved in the most geologically stable parts of the earth's crust.
The formula at the end of each tablet translated as:
We've left you as much of our science as we can. Also our history: if you're reading this we made some mistakes. Please learn from them.
P.S. You're probably really wrong about geology and planetary formation. Sorry. We wanted to set you up for easy bootstrapping.