Graham Hancock is no stranger to controversy. The former journalist, whose books have sold five million copies in the past 10 years, has repeatedly dared to challenge scientific shibboleth, taking a run at entrenched thinking in archeology, geology and astronomy.

His central thesis, laid out in Fingerprints of the Gods and Heaven's Mirror,is that a lost, seafaring civilization, both highly cultured and scientifically advanced, preceded what we now regard as our earliest known communities in Sumer, the Mediterranean and the Indian subcontinent. How else to reconcile these otherwise primitive societies with their extraordinary inheritance -- a legacy of sophisticated astronomical, cartographical and mathematical understanding?

Now, in the vast, 743-page Underworld, The Mysterious Origins of Civilization (Doubleday), Mr. Hancock takes the argument a long step farther.

His central premise is widely accepted: The great meltdown that accompanied the end of the last ice age (7,000 to 17,000 years ago) inundated about 65 million square kilometres of habitable land -- roughly the size of South America and the United States combined.

There is less consensus on how rapidly the mega-melt occurred. Mr. Hancock cites geological evidence that deglaciation occurred not gradually, but in three wrenching and violent cataclysms -- a tidal catastrophe sufficient to raise sea levels 130 metres, abruptly change the geography of the planet and erase, more or less instantly, whatever low-lying cities that existed.

In the antediluvian world, he suggests, Sri Lanka was attached to southern India, Malta to Sicily, via a 60-kilometre land bridge, and the Persian Gulf was essentially dry. After the floods, the Earth's inventory of islands experienced a dramatic increase.

"I've tried to take this forward by correlating the science" -- including work by University of Alberta geologist John Shaw -- "to the flood myths that turn up in cultures around the world," Mr. Hancock said this week in Toronto.

And there is a correlation: In areas that were clearly submerged by deglaciation, popular myths describe the event and its aftermath -- the retreat to higher ground by survivors and the renaissance of communal life. Many stories also allude to a civilization of stature that was suddenly erased.

Of course, for many academics, such myths are exactly that, legends that have no basis in historical event. But Mr. Hancock maintains that the ubiquity and similarity of such myths in so many widely dispersed cultures attest to a universal memory of a genuine flood so vast and so destructive as to constitute a root saga of early human history.

In the past, Mr. Hancock's assumptions and extrapolations have exposed him to sharp ridicule. The scientific community has excoriated his methodology, denounced his conclusions and dismissed him as a circus act. A few years ago, BBC2's Horizons series aired a two-hour dcoumentary that, he charged, tarnished his work and his reputation; he appealed its findings to a BBC ombudsman, who ruled in his favour.

In Underworld, Mr. Hancock has tried to cloak himself more carefully in the recognized vestments of traditional science. The work of respected experts is sprinkled liberally through the text, and the book is more deferential to scientific authority and less extravagant in making claims.

But the general proposition is still liable to make waves (no pun intended) -- that buried in our oceans, largely ignored, are the remnants of once-great civilizations, a lost chapter in human history. If modern homo sapiens, as science insists, have been here for 120,000 years, but our traceable family tree goes back to only about 10,000 BC, what happened in the interval? Nothing? We were hunter-gatherers for 110 millennia?

Mr. Hancock's own efforts have turned up "a handful of genuinely mysterious ruins . . . that cannot be explained within the established model of prehistory. All were last above water at the end of the last ice age and all are too large and complex to have been made by any known culture of that period."

But do they constitute the smoking gun -- definitively Paleolithic (before 10,000 BC), certifiably man-made, reliably carbon-dated structures or artifacts?

"There are things that definitely do not fit in with the historical picture," Mr. Hancock contends. "Structures off the coast of Japan at Yonaguni, and off the eastern coast of India, near Poompuhur, are certainly enough to raise fruitful areas of further inquiry."

At Yonaguni, there are parallel megaliths, stone circles, ramps and other rock formations that may suggest human design -- the jury is still very much divided. But you can see the designs clearly in the book's photographs, taken by Hancock's wife, Santha Faiia.

At Poompuhur, facing the Bay of Bengal, Indian divers found a horseshoe-shaped object, measuring 85 metres in length, in water more than 23 metres deep. According to one scientist, the land on which this structure was built last stood above water more than 11,000 years ago.

In January, Indian marine scientists discovered what may be the more extensive remains of two ancient cities in the Gulf of Cambay. The site spans an area of about 25 square kilometres, 35 metres deep, which, until as late as 6,900 years ago, was entirely above water. About 2,000 possibly man-made artifacts have been dredged and carbon-dated from 8,500 to 9,500 years old.

Mr. Hancock suggests that a comprehensive attempt to mine the oceans archeologically would validate his hypothesis. But he personally is finished with this chapter of his life, at least for now. "I'm really tired of the personal venom. I find elements of it extremely insidious. I'm moving on" -- notably to new books about the origins of humankind and (with Robert Bauval) about sacred myths hidden embedded in the design of Paris, Washington and other cities.

As for the underworld, he hopes that oceangraphic institutes will continue to explore. It's expensive, he concedes, "but it's doable. And if you were going to look for lost civilizations, the oceans are where you'd look."

Michael Posner is a Globe and Mail arts reporter.