The back of Underworld has various press quotes about Graham Hancock, one of which describes him as the "Indiana Jones of alternative archeology." I think the Copernicus is more accurate.
Graham Hancock is often tarred with what you might call the "Jesus is an Alien" brush. In bookshops you'll find his books grouped alongside authors who claim that aliens built the pyamids, that the descendants of Jesus are alive today in a secret society, that the "templars" had esoteric knowledge that they can trace back to ancient Egypt etc.
In fact you will usually see him in the same section as authors writing about alien abductions, or someone like former BBC sports presenter David Icke who claims that the people who run the world are all giant lizards!
Hancock doesn't believe that aliens created civilization and whatever his views on Jesus, it's not a period of history that he writes about.
What he tries to prove in all his books is something that's both more conventional and potentially more exciting: Civilization is much older than we think it is and didn't just evolve out of thin air around 3000 BCE.
Hardly a theory that's in the same bracket as alien abductions or giant lizards when you consider that the Noah story of a great flood that destroyed civilization thousands of years ago isn't exclusive to the Judaeo-Christian tradition. It's shared by societies as far apart as native americans and aboriginal australians.
And as Hancock goes to show in Underworld, ice age earth had a fifth more land than it does today. The Persian Gulf and large areas of the Indian coast were not only land, but were temperate, warm and completely fit for human habitation. (And so was the Sahara incidentally)
Using geological evidence he shows that a number of apparently man-made structures below the oceans could only have been built thousands of years before when civilization is first thought to have started.
And though he doesn't believe that the ancients drove cars or flew planes he does believe that they knew a great deal about astrology, mathematics and linked to that, sophisticated building techniques.
This is a serious, 700 page tome where Hancock takes you through the evidence and all of his thought processes in so much detail that you wonder whether some of his critics have actually bothered to wade their way through it.
Those of us in the UK who saw the Channel4 series that accompanied the book could see for ourselves that Hancock isn't some lunatic but a perfectly sane individual who has studied the alternative points of view as well as the evidence.
Just like it took a while for the flat-earthers of middle ages Europe to come around to the fact that the World was actually round, I'm convinced that Graham Hancock has kick-started a process that will cause us to reevaluate some of our assumptions about history.
And to paraphrase a famous saying, the key to understanding our present and our future is to understand our past.