"Pesticides. Flame retardants. Mercury. These are just a few of the chemicals that whales are exposed to as they swim about in the ocean. How do we know this? Their ears told us. Specifically, their ear wax. It turns out that blue whales are constantly generating new layers of wax in their ear canals. This builds up into an "earplug" that last the whale's entire lifetime. And similar to the rings inside a tree's trunk, scientists have long used the waxy layers of these plugs to determine a whale's age." (Quoted from article)