This is one of the reasons why I hike. I enjoy coming across unexpected sites like these double lean-tos. I found the structure in the photo above on an unmaintained ridge trail in Stone State Park. I suspect it was used for an overnight stay since there were campfire coals and beverage containers nearby.
The double lean-tos in the photos above and below were discovered at Lake MacBride about a mile into the Field Campus Trail. There are many more in this area and I suspect these are the result of a scout group or University of Iowa recreation project.