The smell of rain on dry ground. More specifically, itís the pleasant smell that often accompanies the first rain after a long period of warm, dry weather in certain regions. It was named by two Australian researchers in an article in Nature in 1964, who discovered that the smell is an oily essence that comes from rocks or soil that are often (but not always) clay-based. The oil is a complicated set of at least fifty different compounds, rather like a perfume. It turned out that the oils are given off by vegetation during dry spells and are adsorbed on to the surface of rocks and soil particles, to be released into the air again by the next rains. The word comes from Greek petros, a stone, plus ichor, from the Greek word for the fluid that flows like blood in the veins of the gods.