Hi Lindsay,
I think you might have seen something like the show called Ancient Aliens, that used to be on the History Channel, or a similar show on another cable network. The problem with television shows or documentaries like this is that they often ignore evidence from mainstream scientists and professionals who do research for universities, museums or planeteriums, and government laboratories. When a scientist wants to publish a paper in a reputable scientific journal, it first has to be
reviewed by other scientists to make sure the work was done properly and
that the evidence supports the conclusions. Unfortunately, anyone can publish a book or make a web site with their own personal theories about extra-terrestrial visitors to the Earth, and then go on a television show claiming to be an "author" or an "expert." These books and web sites are generally not reviewed by scientists before publication, so no one checked that the material was properly researched, based upon facts, or that their theory is even plausible.
Just because something is unexplained, does not mean that space aliens are the answer. Anthropologists have researched ancient sites around the world for years to understand why they were built, how they were built, and what purpose they served. Sometimes, there is not enough archaelogical evidence to come up with a definitve answer. It is particularly hard when an ancient society did not leave written records we can understand, or did not leave written records at all. However, as new sites are uncovered and new technologies are developed, we learn more and move closer to plausible explanations. Just because an ancient drawing or sculpture looks like an astronaut to us, does not mean that is what it looked like to the people who created it hundreds or thousands of years ago. We cannot judge ancient societies using popular cultural references from today's society. Thousands of years from now, anthropologists might be really perplexed by elements of our own culture.
Some of the mysterious sites that were once unexplained and are often featured on shows about ancient visitors from outer space, now have plausible scientific explanations. For example, on these TV shows, the Nazca Lines in Peru are often portrayed as being related to extra-terrestrials because we did not know how or why they were created. TV shows often claim that the lines are only visible from the air, which means they had to be made by space aliens since ancient humans did not have aircraft. However, it is not true that the lines are only visible from aircraft. The lines were actually discovered by archaelogists hiking in the surrounding foothills. The lines were probably made by humans using simple tools and surveying equipment, which has been demonstrated by scientists using tools similar to what the ancient inhabitants of Peru had. We still don't know why the lines were made. There have been many attempts to link the lines to constellations or the rising and setting of various astronomical bodies, but these theories have been difficult to prove. Another theory is that the lines were related to the worship of gods related to water and crops.
It's fun to think that there could be other planets with intelligent life out there, but you need to be careful about believing everything you see on TV. When you watch documentaries, it is important to think carefully about the information presented and the credentials of the people being interviewed. It is also important to consider who made the documentary, and if they have a reputation for providing accurate information, or if the documentary was made by people who are more concerned with getting high numbers of viewers than presenting facts.
Kris