Chances are, we have all ‘smelt’ natural gas. It is not a bad, offensive smell but it is certainly not pleasant and because of this smell, we know that there is unburnt gas about. Not a welcome prospect in a Chemistry lab! But did you know, natural gas (or methane) is actually odourless?
Image kindly reproduced according to the licence at: http://en.wikipedia.org/wiki/File:Methanethiol-3D-balls.png
This is because methanethiol (also known as methyl mercaptan), another of our smelly sulfur compunds is added to natural gas. Why is it added? Well, so that you are able to tell (by its smell) if there is a gas leak!
Methanethiol is often referred to as smelling of rotten cabbage. It is found naturally in nature and it is often released from decaying organic matter. It is also a byproduct of apsparagus metabolism. It was once thought that this was a genetic trac but it has know been found out that the ability to detect the odour is a genetic trait and not the ability to make the bad smell!1
1 Article adapted from an article found at: http://en.wikipedia.org/wiki/Methanethiol