A rotary windmill
Ed. note: I have been schooled on wind things. This is a "cowled turbine," not a rotary windmill - no Dutch people in wooden clogs were harmed in the making of this interesting structure.
Deer Island is a fascinating place to walk around. Look outward as you walk along the path, and there are great views of the water, from Boston Harbor to the open ocean.
Of course, it's hard to ignore the giant sewage-treatment plant, especially the giant "eggs" that house the sewage-munching bacteria.
The paths are open year round, even on days like today.