"One of the strangest mysteries [about Stonehenge] is why those giant stones were dragged to an unremarkable Salisbury hillside instead of being erected where they were originally found. Now it seems we have an answer - some of them were already there." Source: Science Alert (April 2018).
"Archaeologist Mike Pitts - one of few researchers to ever perform excavations within Stonehenge - has found evidence that two of the monument's biggest and most important stones, called sarsens, may have been at the site for millions of years."
Comment: I was kind of surprised when I read this story. I thought it was well established that the stones used to build Stonehenge were somehow transported to the site, apparently over a long distance, and that archaeologists / geologists had identified the areas where the stones were quarried. The main question was how these gigantic stones were transported and people have been coming up with various theories over the years.
Now it turns out that the stones (or at least some of them) may not have been transported from somewhere else at all. This makes a lot of sense, actually. According to Occam's Razor, when you're faced with a problem, the simplest solution tends to be the right one. In the case of Stonehenge, if nobody can figure out how Neolithic people could have moved these stones over such large distances, the simplest answer is that they didn't have to move them because they were already there.
Even if this is true, however, it still doesn't solve the problem of how the stones were shaped, raised and positioned. One way or another, Stonehenge is a masterpiece of prehistoric engineering and the absence of written records means that we'll never know for sure how it was constructed. There are only a limited number of ways it could have been done, though, so the ultimate answer will probably just be a list of possible techniques.