Speculating on ancient mysteries is good fun - the jaw-dropping megalithic constructions of cultures around the world often inspire awe, and attempting to solve the puzzle of how and why they were built can lead to both fresh insights into human history, and sometimes not-so-useful ones (though exploring these cul-de-sacs can sometimes be helpful in other ways). The History Channel series Ancient Aliens anchors much of its premise - that aliens visited the Earth in ancient times, and inspired or built these structures themselves - on megalithic mysteries. But how much of AA's claims are solutions to puzzles, and how many are intellectual dead-ends?
A new 3-hour(!) documentary, Ancient Aliens Debunked, takes the series apart, fact-checking individuals claims on topics including pyramid construction, the Temple of Baalbek, Incan sites, Easter Island. Pacal's "rocket", the Nazca lines, and the Egyptian "light bulb". Produced by Chris White, and made available freely online (embedded at the top of this post), Ancient Aliens Debunked offers clarifications, corrections, and background information that certainly removes much of the superficial gloss from Ancient Aliens.
Ancient Aliens is an American television series which premiered on April 20, 2010, on the History Channel. The program presents the so-called 'ancient astronaut' theory, which is the idea that extraterrestrials visited the Earth in the ancient past, and that historical texts, archaeological records and various legends contain evidence of this contact. I used to believe that the ancient astronaut theory was true. I spent years learning about it from the popular authors, from video presentations and radio shows. I was an enthusiastic promoter of this idea to my friends and family. Although I no longer feel that way, I want you to know that I have no personal reason to reject this theory. My current worldview could easily accommodate the existence of extraterrestrials. I hope you will come away from this film, even if you disagree with it, saying that I fairly reviewed the claims of Ancient Aliens without bias, and with respect.
I think it's a shame actually that the title of the documentary includes the word "Debunked" - it's a negative word, guaranteed to set many of those interested in this topic at odds with the film before they even view it. In reality, there is a wealth of information about ancient construction and artworks that most open-minded people will appreciate greatly. Additionally, the documentary doesn't shy away from noting actual mysteries, and speculative attempts to solve them, such as the theory of French architect Jean Pierre Houdin on how the Great Pyramid was built. A far better title for the film (IMHO) would have removed the negativity and focused on the positive aspects of what is a very well-researched, informative piece on ancient mysteries - something along the lines of "Exploring the Claims of Ancient Aliens". In short - don't be turned off by the title, it's well worth your time.
Also: if the full 3 hours is too much for you, you can watch (or read transcripts of) individual 'episodes' on separate topics ('The Pyramids', 'Easter Island' etc) at the Ancient Aliens Debunked website, where you'll also find references for each rebuttal for further research. An excellent - I might even say necessary - resource for anybody interested in this fascinating topic.
Addendum: As SecretSun points out in the comments, the producer of this documentary, Chris White comes from a fundamentalist Christian point-of-view, with the added colour of anti-Christian conspiracy theories. However, the documentary above is almost completely free of these elements (you will notice some strange flavours in sections such as 'The Nephilim'), and is presented in a very sane manner - and I can only judge it by its content. But it is worth keeping in mind that all presenters - be they Christians, 'skeptics', or ancient alien theorists, are working within a frame of reference, and often pushing a barrow of their choice. So as always, I recommend that you keep your wits about you and employ skepticism, objective judgement, and further research in understanding any topics presented on this site.