It’s the first smell of death I’ve encountered today and it’s instantly recognisable. Denis Johnson wrote about this smell, in a story called Triumph Over the Grave: he said that ethyl mercaptan, the first in a series of compounds brought out in the process of putrefaction, is routinely added to gas to make leaks detectable by scent. The practice originated in the 1930s, after workers noticed vultures in California would circle the thermal drafts around leaks in pipelines. They ran tests on their product to see what had attracted these birds, ordinarily lured by the odour of decay, and found trace amounts of this compound.