Fat Deer Key is an island in the middle Florida Keys.
Map of Fat Deer Key, Florida 33050, USA
U.S. 1 (or the Overseas Highway) crosses the key at approximately mile markers 53.5-56, between Long Point Key and Key Vaca.
It is entirely within the cities of Marathon, Florida and Key Colony Beach, Florida.
It has the only road leading to the city of Key Colony Beach, known as Sadowski Causeway.
References
Fat Deer Key Wikipedia(Text) CC BY-SA