Here is a site which covers more than 300 miles square, and may have taken thousands of years to build, but because it is in Africa, we don’t know about it. So far as I know, National Geographic has never sponsored any major research on it. I have spent hours on Google Earth going all over this huge area, and from erosion and sand coverage my guess is that it covers thousands of years of development at various times. I wonder if the civilization at Greater Zimbabwe was created by the people who used to live at those channels. There are several other links with other photos, but looking down on them yourself is the most revealing.
http://earthepochs.blogspot.com/2014/04/the-largest-man-made-system-on-earth.html
They are natural formations, but they are pretty neat, and it comes up once in a while.