Yes, this really is just a mile marker sign, just like the 100- or so others you've counted on throughout the Keys to give you your position along the chain of islands. Only this is the last or first one, depending on how you look at the world. Mile Marker 0 represents the end of the line, the furthest south you can get from the lower 48 continental states, and still be in the USA. It's the end of Highway One, which starts way up in Maine. It represents paradise, and a slower pace of life, and it represents that you've arrived, you made it. Definitely get your picture taken underneath the sign, if somebody hasn't stolen it recently, which happens a lot. It's a crime to mess with the sign, and there's no need: you can buy signs, postcards, hats, almost anything with MM0 on it from the shops in Key West.