Although some are undoubtedly man made, many others are most certainly not.
Can we have a link to a photograph of one of the latter please?
There's some very interesting information that can be cross referenced on this particular site. The problem is, finding interesting/serious data etc is quite laborious due to the sheer number of sites that include it amongst the more spurious fringe element material. In my opinion it's a fascinating area, if approached with patience, clear thinking and an open mind.