The smell of rain (petrichor) is largely geosmin, a signal made by soil bacteria that humans are extraordinarily sensitive to
The earthy smell after rain, named "petrichor" in 1964, is largely one molecule — geosmin — released by soil bacteria (Streptomyces) as they form spores to attract small soil creatures that spread those spores; the human nose can detect it at less than a part per billion.
Sources
A. The earthy scent of rain on dry ground was named "petrichor" in 1964 by Bear & Thomas in *Nature*.
Wikipedia, "Petrichor" (fetched 2026-07-26) — "Thomas coined the term 'petrichor' to refer to what had previously been known as 'argillaceous odour'." Cites I. J. Bear & R. G. Thomas, *Nature* 201 (4923): 993–995 (March 1964).
The Scientist, "An Earthy-Smelling Substance, 1964" (the-scientist.com, fetched 2026-07-26) — independent secondary confirming Bear & Thomas 1964 *Nature* coinage.
B. A large part of that smell is geosmin, a compound produced by soil bacteria of the genus *Streptomyces*.
Wikipedia, "Petrichor" — "the *Streptomyces* genus of gram-positive bacteria, have, in part, been implicated in producing the chemicals." Geosmin = "earth smell."
Nature Microbiology, Becher et al. 2020, vol 5, 821–829 (nature.com/articles/s41564-020-0697-x) — "*Streptomyces* bacteria make volatile compounds such as geosmin and 2-methylisoborneol." Note: "a large part of," not "all of" — petrichor also includes plant-derived oils and ozone (skeptic note below).
C. Streptomyces make geosmin specifically when sporulating, and it attracts springtails (Collembola) that disperse the spores — i.e. the scent is a signal.
Nature Microbiology, Becher et al. 2020 (primary) — geosmin/2-MIB synthase genes are "under the direct control of sporulation-specific transcription factors"; the volatiles "attract a soil arthropod (F. candida)" which spreads spores "via faecal pellets and through adherence to its hydrophobic cuticle," spores surviving gut passage.
phys.org, "Research unearths the science behind the smell of spring" (2020, fetched 2026-07-26) — independent secondary: "Soil gets its smell from bacteria trying to attract invertebrates."
D. Humans detect geosmin at extraordinarily low concentration — below one part per billion.
Wikipedia, "Petrichor" (citing Polak & Provasi 1992) — "the human nose is sensitive... to geosmin, and can detect it at concentrations as low as 0.4 parts per billion."
Water-quality authorities (Zone 7 Water Agency; City of Kennedale geosmin FAQ, fetched 2026-07-26) — human detection threshold ~5–15 parts per trillion. Independent corroboration that the threshold is well below 1 ppb.
What we refused to claim
The exact detection number. Sources spread from ~5 ppt to 0.4 ppb. The script asserts only "less than a part per billion," which is true under every source cited (all agree it is sub-ppb). No single hard number is spoken.
"15 million times more sensitive than a shark is to blood." A derived/embellished comparison found only in secondary blogs, not in a primary source. Dropped entirely.
"We evolved to smell it to find water." Plausible and repeated, but not firmly established. Script says only "you just happen to be able to overhear it" — no evolutionary-purpose claim.
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