Stonehenge

The only thing we can be sure of is that the stones were not moved by aliens. We can only see it through our 21st technology. If the wheel hadn't yet been thought of, how would we do things? The sea route is plausible as people did travel a lot more than we think they did. The question is, what sort of a boat did they use? As for the overground route, there must have been a technology that we haven't found yet.