Is Belfast's other 'unsinkable' ship about to give up its $1bn treasure from the deep?

But it's not the Titanic that Martin Bayerle is obsessed with, rather its long-forgotten sister Republic which, the American entrepreneur believes, was carrying 150,000 US 'double eagle' gold coins - worth in excess of $1bn in today's market - when it sank in 1909.

Bayerle's attempt to recover the precious cargo, which was apparently intended for delivery to Russian Tsar Nicholas II, is now being chronicled on television by The History Channel.

He first tried - and failed - to locate the gold nearly 30 years ago, going bankrupt in the process.

Now, with salvage rights to the ship secured and having raised millions from venture capitalists, the New Yorker is convinced he'll pull off the world's largest-ever treasure recovery.

"In 1987 my research was incomplete, and we spent 74 days excavating in the wrong place," admitted the 64-year-old father-of-two, whose previous expedition merely uncovered the ship's wine cellar.

"Now, after three decades of additional research, we have verified exactly where the treasure is. We have good tools, we have good people, and now we just have to go for the gold."

RMS Republic, loaded with wealthy passengers bound for the Mediterranean, was lost off the coast of Nantucket Island, Massachusetts, on January 23, 1909, after being hit by an Italian steamship which had lost its way in dense fog.

Of the 1,742 people on board the 580ft, 16,000-ton White Star liner built by Harland and Wolff in 1903 - only six died, but all the baggage and cargo went down with the doomed vessel to its final resting place, 270ft below the surface. At the time, she was the largest ship ever lost in maritime history.

The gold was, apparently, a loan to Russia from the US government - something the White House was keen to keep secret.

"The coins were worth 20 dollars each at the time when people were earning six dollars a week," said Bayerle, who added that he wonders whether the sinking of the Titanic three years later might have had a different outcome if the Republic's construction flaws were given more attention. "Both ships were built at the Harland and Wolff shipyard in Belfast with the same design," he said.

"They were sister ships, termed 'practically unsinkable.' But they both sank. I can see, though, why there was no panic aboard Titanic. The loss of Republic was big news. Everybody knew about it.

"But over 18 hours, all the passengers, except of course the three passengers and three crew who died at the point of collision, were safely off-loaded."

In his book, The Tsar's Treasure, Bayerle - who first discovered the wreck of the Republic in 1981 - says the ship's manifest included both the Russian gold shipment and a US Government in-coin currency shipment, valued at a collective $3,800,000 in 1909. The failure of the first expedition hit Bayerle hard, and in the early '90s he was sentenced to five years in prison for voluntary manslaughter after shooting dead his estranged wife's boyfriend.

He served half the sentence and then set about securing salvage rights to Republic.

Billion Dollar Wreck is being shown on Mondays on The History Channel.