I saw this article at BoingBoing, and had to look it up.

From Wikipedia:

The Aroma of Tacoma, also known as the Tacoma Aroma, is a putrid and unpleasant odor associated with Tacoma, Washington, United States.[1] The smell has been described as similar to the odor of rotten eggs.[2] The odor is not noticeable throughout the city, but is rather concentrated in the Tacoma tideflats and is frequently smelled by motorists traveling that section of Interstate 5.[2]