No two people in my experience have contributed more to our understanding of the Titanic and its tragic sinking than Ken Marschall and Don Lynch. Ken's luminous artwork, based on his unparalleled knowledge of the ship, and Don's encyclopedic mental database of her crew and passengers make them unrivaled Titanic authorities.

For this reason, I asked them in 1995 to act as advisors on my film Titanic. Not only did they help me get it right, but we also became friends in the process.

When I began planning a return expedition to the Titanic in 2001, 1 enlisted their aid again. This time, the objective was not merely to photograph the exterior, as we had done during our 1995 expedition, but to use sophisticated remotely operated vehicles (ROVs) to do a thorough imaging survey of the interior of the wreck.

My hope was that Don could help me identify which passengers had been in which cabins and prioritize the list of historically interesting targets. Ken, meanwhile, could bring his vast knowledge of the wreck to bear in planning ROV penetrations of the various deck levels, cargo spaces, and so on.

After our initial planning meetings, it quickly became dear to me that these two Titanic experts would prove invaluable on the expedition itself — so I offered them the chance to come along. And I'm grateful that they both accepted.

Ghosts of the Abyss, the film that resulted from the 2001 expedition, dearly demonstrates their contribution to the mission, and we certainly could not have accomplished what we did without their participation.

Ironically, neither Ken nor Don was particularly interested in diving to the wreck itself Many people are daunted by the prospect of being locked up for sixteen hours in the cramped manned sphere of a deep-sea submersible (a space about six and a half feet in diameter) with 125 million pounds of water pressure squeezing the sphere from the outside as it crawls along the vast ocean floor searching the pitch darkness with sound waves instead of vision. It's definitely not for the faint of heart.

Add to this the hazards of wreck diving — turbulent currents flowing around the wreck that can spin the submersible out of control or twisted steel that can snag and trap the sub — and you find that most sensible people decline the offer.

But as the expedition progressed, I could see that both Ken and Don were becoming more and more fascinated by the diving itself. And it was dear that they were finding it hard to resist the strong pull that a dose-up look at the wreck had on them. Finally, a week into the dives, both of them made the final plunge — literally!

Ken eventually made four trips down to the Titanic, and Don made two. They became hardened sub divers, adept at underwater navigation and communication, and were of great assistance to me in planning and in actually performing the very complex operations on the bottom. Ken's ability to find his way around the wreck was uncanny.

I might be piloting the ROV Jake deep inside the wreck, down on the D-deck level, and I would ask Ken in the other submersible to move to a specific porthole — say, D-35 on the starboard side — and shine the sub's spotlight inside. Within minutes, Ken would have directed the sub pilot to the exact window and the light would come bursting in like a sunrise. It was moviermaking magic.

Diving with Don was also a great experience. We might be looking at a spot on the Boat Deck, or at a window or a davit, and Don would always know exactly what had happened there. He was able to tell the story of the Titanic's passengers and crew right there - right at the very spot it happened.

To be in that alien place, exploring this vast human artifact, and to have these two historians bringing it to life with their intimate knowledge of the event was for me the pinnacle of underwater exploration. I can only guess what the experience was like for them. In Don's case, to actually witness and interact with the ship that had dominated his work and thoughts for decades must have been amazing.

And Ken's endless curiosity about the wreck must have been satiated and simultaneously whetted even further by the images that the two intrepid little 'bots were able to bring back.

Together, we were able to explore many of the interior spaces of the wreck — though others remained tantalizingly out of reach. With the help of the specially designed 'bots, we imaged areas that had never been photographed before and finally found out what they looked like. In 1912, no one expected that the ship would live only a few days, so it was always assumed there would be plenty of time to photograph her inside and out.

The Titanic went to the bottom with many secrets, and we were able to unveil a few of them. Thanks to Ken and Don's systematic analysis of the images we retrieved, we now understand so much more of the interior architecture and design of the Titanic, as well as the events of the sinking.

I was on the bottom of the ocean, at the wreck site along with Ken and Don, when the attack of September 11 took place. We surfaced into a changed world. Our sense of isolation was intensified because we were stuck out in the middle of the North Atlantic, hundreds of miles away from the unthinkable tragedy that had occurred at home — and away from our families and friends. At the same time, everyone on board was drawn closer together, in a bond I suspect will be lifelong.

The 9/11 attacks also made us reassess what we were doing at the Titanic site — and why. Here we were, poking through the wreckage of the defining disaster of the early twentieth century, while the defining disaster of a new century had just taken place.

At first, our passionate study of the Titanic wreck seemed suddenly pointless and trivial — but in the following days, it took on a new meaning. It became a way for us to talk about tragedy and loss, and about the shock and numbness caused by events that seemed out of all human proportion.

The Titanic was a "safe" tragedy from another century, and we used it to focus our emotions. It helped us come to terms with what was happening in our world now - and perhaps that is ultimately the reason for our collective fascination with the Titanic.

The disaster has always been the quintessential story of loss, of coming to terms with death, of heroism and cowardice, and the full spectrum of human response before, during, and after a crisis.

As such, it will always be with us as one of the great lessons of history. And its mythic status will endure.

Ken Marschall and Don Lynch are among the most gifted bearers of the Titanic story, and this book illuminates and continues the legend of the mighty ship.

- James Cameron