Everyone knows the smell: rain hits dry ground and something rises, earthy, sweet and strangely comforting. In 1964 two chemists at the CSIRO in Melbourne, Isabel Joy Bear and Richard Thomas, worked out where it comes from. They steam distilled rocks and clay that had baked in the sun and found a yellowish oil that stone and soil hold in their pores through dry weather and give up the moment water arrives. They named it petrichor, from the Greek for stone and for the golden fluid that ran in the veins of the gods, and published it in the journal Nature on 7 March 1964. Later they showed the same oil holds seeds back from sprouting during a dry spell, so that a plant waits for real rain before it commits. Mixed into the oil is a second scent, geosmin, and that one is alive. It is made by Streptomyces, the soil bacteria that have given us most of our antibiotics, and they only make it when they are ready to spore.

Why would a bacterium bother with perfume? In 2020 Klas Flardh at Lund University and Paul Becher at the Swedish University of Agricultural Sciences showed that geosmin is a dinner bell. It draws springtails, tiny six legged soil creatures that have been on Earth for about 400 million years, which eat the bacteria and carry the spores away on their bodies to new ground. Rain is what throws the whole mixture into the air. At the Massachusetts Institute of Technology, Cullen Buie and Youngsoo Joung filmed raindrops landing with high speed cameras in 2015 and saw each drop trap a fizz of tiny bubbles that burst up through it, releasing hundreds of scented droplets in a few millionths of a second, and they found that light and steady rain does this best, while a downpour barely does it at all. Our noses are tuned to the result, a few parts per trillion is enough, which is why the first shower after a dry spell reaches you before the first drop does. Whatever the season where you are, the next time rain begins, that smell is stone giving up its oil, bacteria calling their friends, and a burst of bubbles doing the delivery. It is hard not to love it a little more, knowing that.

Our short take: petrichor's naming from the CSIRO's account of Bear and Thomas (first published in The Conversation, March 2015) and CSIROpedia; the springtails from Lund University's release on the 2020 Nature Microbiology study by Flardh, Becher and colleagues; the raindrop bubbles from MIT News on the 2015 Nature Communications study by Joung and Buie.