SMELL
There's a smell after it rains
There's a smell after it rains, and it has a name — but it was never meant for you.
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." 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."
BACTERIA
made by tiny bacteria that live in the soil
A large part of that smell is geosmin, a compound produced by soil bacteria of the genus *Streptomyces*. Humans detect geosmin at extraordinarily low concentration — below one part per billion.
Wikipedia, "Petrichor": "the *Streptomyces* genus of gram-positive bacteria, have, in part, been implicated in producing the chemicals." 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." Wikipedia, "Petrichor" (citing Polak & Provasi 1992): " (citing Polak & Provasi 1992) — "
SPORES
They release it as they form their spores
The earthy scent of rain on dry ground was named "petrichor" in 1964 by Bear & Thomas in *Nature*. A large part of that smell is geosmin, a compound produced by soil bacteria of the genus *Streptomyces*. Streptomyces make geosmin specifically when sporulating, and it attracts springtails (Collembola) that disperse the spores — i.e. the scent is a signal.
Wikipedia, "Petrichor" (fetched 2026-07-26): " (fetched 2026-07-26) — " The Scientist, "An Earthy-Smelling Substance, 1964" (the-scientist.com, fetched 2026-07-26): "An Earthy-Smelling Substance, 1964" Wikipedia, "Petrichor": "the *Streptomyces* genus of gram-positive bacteria, have, in part, been implicated in producing the chemicals."
GEOSMIN
you can smell geosmin at less than a part per billion
A large part of that smell is geosmin, a compound produced by soil bacteria of the genus *Streptomyces*. Humans detect geosmin at extraordinarily low concentration — below one part per billion.
Wikipedia, "Petrichor": "the *Streptomyces* genus of gram-positive bacteria, have, in part, been implicated in producing the chemicals." 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." Wikipedia, "Petrichor" (citing Polak & Provasi 1992): " (citing Polak & Provasi 1992) — "
RAIN
So the smell of rain is really a message between the earth and its smallest life
So the smell of rain is really a message between the earth and its smallest life — and you get to overhear it.
The earthy scent of rain on dry ground was named "petrichor" in 1964 by Bear & Thomas in *Nature*. A large part of that smell is geosmin, a compound produced by soil bacteria of the genus *Streptomyces*.
Wikipedia, "Petrichor" (fetched 2026-07-26): " (fetched 2026-07-26) — " The Scientist, "An Earthy-Smelling Substance, 1964" (the-scientist.com, fetched 2026-07-26): "An Earthy-Smelling Substance, 1964" Wikipedia, "Petrichor": "the *Streptomyces* genus of gram-positive bacteria, have, in part, been implicated in producing the chemicals."
Next time it rains, step outside and take one slow breath of the wet ground — and really, actually smell it.
Next time it rains, step outside and take one slow breath of the wet ground — and really, actually smell it.
“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.
Publishing what we refused to claim is the strongest evidence the rest was checked.