The Seven Mile Bridge is the signature crossing of the Middle Keys, stretching over blue water between Marathon and the Lower Keys. The modern highway bridge carries traffic; the older railroad bridge section beside it has become a walking and cycling route to Pigeon Key after restoration.
It is worth stopping rather than simply driving across. From the old bridge you get a slower view of the water, the weathered rail-era engineering, and the scale of the ambition that once carried trains all the way to Key West.