"Is that dirt under your arm-pit?".
 

My birthmark is located under my left-arm, in the arm-pit area, is brown in colour, is about 1/2" inch across, and is called a "Crab Nebula".

Most of time I am not self-conscious of it and I am unaware of it being there unless I see it in the mirror...

Most people who see it do not comment on it in public-bathing areas or at swimming-pools or when I am shirt-less...