This is partly about sampling the microbes in caves and finding that they're sparse but surprisingly diverse, and some chemistry about there not being enough carbon dioxide in ground water to make limestone caves unless more CO2 is added by microbes, and silica rock being too resistant to mere water for them to form caves without some help from biology.
I liked the podcast a lot-- it's not full-on tech talk, but it's a little above what I can follow easily (need to know more about chemistry!), which makes it feel more solid than a TED talk. And it's not about politics or other painful subjects.
What's more, not only are there 50 more This Week in Microbiology talks at the site, but there's also This Week in Parasitology and This Week in Virology.
Link thanks to albatross at Making Light.
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