When the Franklin Park Zoo expanded, the bears got some new enclosures, but taking down bear-proof cages is a lot harder than putting them up, the old ones were just left where they were to moulder away in the woods.
Now existing as part of Zoo New England, Boston’s Franklin Zoo in, well, Franklin Park (Boston, New England), has been around since 1912 in some form or another. Originally it was intended to be an open plan zoological attraction designed by Frederick Law Olmsted, who had also designed the surrounding park. When it opened the free zoo was a runaway hit (with thankfully no runaway animals), drawing millions of people a year. Unfortunately, the good times didn’t last and by the 1930s the wild zoo fell into decline.
In 1958, the zoo was taken over by a commission with the city which set out to revitalize the historic menagerie. First order of business was to build a fence around the perimeter of the zoo and begin charging admission. In addition, a more professional staff was hired and the zoo was brought into the modern age. Well, most of it.
Some of the older structures, like the original bear pens (and a badger enclosure) were simply left outside of the fence to rot. Even a plan to tear them down made around the time of the new millennium seems to have been left to collect entropy, so there they sit to this day.
The old metal and stone structures seem a bit dangerous now that they are crumbling and covered in rust. They may have been built to keep visitors safe, but these days, they may be the most dangerous things in the park.