A serious NASA-adjacent thought experiment: if an industrial civilization existed on Earth millions of years ago, would its traces survive in the geological record? Probably not.
Documented
The 2018 paper, dead sober
Published in the International Journal of Astrobiology, the paper enumerates what an industrial civilization actually deposits: carbon isotope anomalies, nitrogen in sediments, plastics, synthetic chemicals, and how little of it persists. The Anthropocene's own spike will be a thin smear in a few million years. The authors' purpose was calibration for finding civilizations on other planets, but the method turns on us: our record's resolution is too coarse to exclude a Silurian-scale precursor.
Assessment: Documented science, carefully framed, explicitly not a claim of prior civilizations. The paper says we would likely miss one, not that there was one. Every honest summary must keep that distance, because the internet does not.
Stories
The reptiles before us
The folk version needs no encouragement: Atlantis with a lab coat, ancient astronauts with tenure, Tartaria in the Mesozoic. The paper's title, borrowed from Doctor Who's subterranean lizard people, handed the folklore its own mascot, and the story now circulates as science admits there may have been a civilization before humans.
Assessment: Documented as instant mythology. What science admitted is a measurement limitation. The gap between cannot rule out and evidence for is the exact territory this archive exists to patrol.
Interesting claims
The interesting edge
The genuinely interesting claim is methodological: the usual rebuttal to ancient-civilization stories is absence of evidence, and the Silurian calculation shows the absence is guaranteed by entropy, not by history. That cuts both ways. It restores humility to the debunkers while giving the dreamers nothing solid to stand on, an artifact, a date, a stratum, which they still have never produced.
Assessment: A documented evidential blind spot, not a documented civilization. The hypothesis's real gift is that it disciplines both sides of the argument at once. Skepticism needs it as much as speculation does.
Sources and further reading
Schmidt and Frank, The Silurian Hypothesis: Would It Be Possible to Detect an Industrial Civilization in the Geological Record?, International Journal of Astrobiology (2018)
The Doctor Who Silurians (1970), the borrowed name
Anthropocene Working Group literature, on how thin our own stratum will be
Worth asking
If absence of evidence is built into the geology, how would you ever prove the negative?