Natural gas in its native state is colorless and odorless. Mercaptan is the additive that is added to natural gas to make it easier to detect in case of a leak. The most important thing to know about mercaptan is that it stinks.
Some people compare it to the smell of rotten eggs.
In a concentrated form, its smell is almost unbearable. And it takes only a few parts per million of mercaptan to give natural gas a smell. That is precisely why we add it to natural gas. If we did not add mercaptan, it would be hard for you to know that unlit natural gas was coming from your stove after you left the valve turned on. And leaks from furnaces and hot water heaters would be nearly impossible to detect without expensive equipment.