Well, one of the answers you seek is pretty mundane and pointless and one of your questions is just a bit too vague (but may have answers).
The crop circles began with a couple of old coots playing pranks. Once they had gotten a lot of attention, copycats sprang up all over. The “there is no way to explain this” dribble was pretty easily debunked (except for those who needed a mystery) when the old coots went out and made some circles for a news crew–leaving exactly the same amount of “no evidence” behind.
Indian tribes? Lots and lots of Indian tribes in New England died of various European diseases brought by earlier explorers (accidentally, in this case) just prior to the landings by the Mayflower contingent and their followers. The diseases and the dead tribes were pretty well recorded by the earliest colonists, but the stories were later played down so that the European invasion could be portrayed as a colonization of empty forests with a few scattered Indians rather than a massive land grab.
The Pequod tribe was pretty thoroughly destroyed in battle with the New England colonists.
The Mohicans were sandwiched between invading whites and angry Iroquois and they didn’t survive, either.
The other Indian-related disappearance story is that of the Roanoke colony. They were a small group living in a swampy area that was not particularly healthy for Northern Europeans. A group of 15 soldiers/explorers who had been left to guard a fort from a 1586 exploration had disappeared (and the fort razed) when the next group of 150 people arrived in 1587. The organizers of the colony took their ships back to England to get more supplies, but were gone for quite a while. (Sixteenth century ships were pretty slow and raising funds for a colony while trying not to get caught up in England’s war with Spain was a hassle.) When the resupply ships finally returned in 1590, there were no people in the abandoned colony. The word “croatan” was found carved in a tree, but no one knows what it means. Since the original 15 men holding the fort apparently died violently, it may be assumed that the survivors of the 150 also were attacked. On the other hand, they may have simply died of disease and hunger. An indian tribe living some distance away was later discovered wearing clothing that resembled late sixteenth century English garb. They may have taken clothing from the dead colonists, they may have offered shelter to the dying colonists and picked up the clothing styles before the colonists finally died, it may be pure coincidence.
Do any of these indian stories ring a bell?