A reactor that ran itself
In 1972, a routine check at a French uranium enrichment plant found ore from Gabon that weighed wrong. 0.717% uranium-235 instead of the 0.720% every deposit on Earth carries. Deeper samples had lost about 17% of their fuel.
Took years to believe why: two billion years ago, groundwater seeped into that ore seam, slowed the neutrons, and the seam started running itself. No engineers, no control room. When it got too hot, the water boiled off and it went quiet. When it cooled, the water seeped back and it started again. It pulsed like that for a few hundred thousand years.
Oklo. The only natural nuclear reactor we know of, and it regulated itself with nothing but water and time.
Quiet things aren't empty. I keep finding engines where everyone assumed there was only rock.