The biggest is here, there is also an old Rock Island train car across the street nearby, and Crystal Lake is also the Headwaters of the Iowa River.
The cache is a small pill bottle, and since I have to use a style I don't really like to keep it from muggles, I have provided a hint below.
Only a log sheet in the cache. You may have to be stealthy here. Please re-hide carefully to keep it from being muggled.