they aren't made by aliens lol, but they are people who have a lot of time on their hands lol
the ones that I have seen when traveling through Ontario are small, and usually on large rocks and boulders a long the highway. read about them on the wikipedia link that I posted above.
I read this morning that some of the parks in Canada have had to ask people to stop building them, because they are building way too many and confusing people in some areas where they actually use them to mark paths.