Everything I know about GIS I've learned myself.
I wrote a small utility for my non-profit employer that used the USGS 'The National Map's online services to pull a sized image of USGS Topographical Quad. I wrote the utility many years ago, and it was not only a crash-course in GIS, but in accessing a WMS.
The URL I had been using was similar to:
..which would give an image of 1000px x 1000px around Taos, New Mexico. (Coordinates and image size would change, depending on user's area of curiosity.)
Now, however, that service was shut down. In my infinite glory, I had completely missed the posted notification and am now stuck without a running utility.
According to the USGS, the service has moved to 'services.arcgisonline.com/arcgis/rest/services/USA_Topo_Maps/MapServer'
So, learning what I could about the service, apparently WMS isn't available and I need to use the ExportMap functionality. This is the new URL I was able to translate the above to:
However, if you click on that, you'll see I get an awesomely helpful error of "bytes", code 500. (As a developer, I know Code 500 is Internal Server Error, of course..) But without a more helpful message, I'm not sure where I bungled it up.