"Key" is actually just the English accent applied to the Spanish word "cayo" meaning nothing more than "island". It is also the county seat for all of the Keys. There is not much in the way of square miles above sea level, but it is over 100 miles to the easternmost edge of Key Largo. There are not many US counties that are 100 miles across. Not even in Texas.

One of the more interesting features of the Keys between the Mainland and Key West is The Railroad that Went to Sea build by Henry Flagler with his own money (one of JD Rockerfeller's original partners).

The opposite (east) end of Marathon is the home of the Dolphin Research Center where, with advance reservations, you can play in the water with the kiddies.