Ever since the History Channel show "Ancient Aliens" premiered, many of the people who are interviewed on the show (mind you they tend to be the same people) assert that not only were we visited by extraterrestrials in the past, but that historians and archaeologist, along with the government, are covering up the evidence.
The belief is, is that the leadership within historian and archaeologist community, along with the government, is hiding this knowledge of ancient extraterrestrial visitation in order to keep the status quo, and to prevent chaos from what might happen if it was revealed that certain histories in many peoples' religions were in fact false.
While some people might question their beliefs if such information did come out, many people would try re-work their beliefs to fit this new information, or they would reject this information outright and claim it to be itself a part of a conspiracy to destroy Christianity. Just take a look at how evolution is viewed in this country. Some people have accepted it, some reject it, and most have work evolution their beliefs, which tends to be called theological evolution.
Many ancient alien "experts" also assert that fear by threats of ruining someone's career is what is mainly used to keep such information suppressed, and that some historians and archaeologists careers have been ruined already, because they would not give up on their belief that aliens visited the planet in the past.
Most historians and archaeologists who's careers have been ruined, didn't get them ruined because they publicly stated that aliens visited us in the past. Their careers were ruined because they wouldn't give up on a theory that has been discredited and proven to be completely untrue. This doesn't just apply to historians and archaeologists who believe in ancient alien visitation, it applies to anything in archaeology that's been disproved.
Besides the fact that evidence they present on that show about ancient alien visitation is highly speculative at best, and is usually completely false due to a profound misunderstanding by people who don't understand history and don't know what they are talking about, they also present no evidence that information about ancient aliens either exists, or is being suppressed at all.
Now I won't say that aliens never visited us in our past, because there is no evidence to support that we never were visited at all in the distant past, but, there is also no evidence to support that we have been visited by aliens in the past either.