Martin Bayerle organized an expedition in 1987 to explore the sunken wreck of the RMS Republic off the coast of Nantucket hoping to find gold. Over $2.5 million was invested in the expedition. Unfortunately, he only recovered wine bottles, plates and a toilet.

Mr. Bayerle went bankrupt, was forced to sell his Vineyard home and move to his in-laws’ farm in West Virginia. He later became estranged from his wife, was convicted of voluntary manslaughter, and served two and a half years in prison. But even while landlocked and locked up, Mr. Bayerle never gave up hope of finding the treasure. He is, after all, the only person deemed a pirate by the U.S. court system — in an affectionate manner, he says.

Mr. Bayerle is currently the subject of an eight-part series on the History Channel called Billion Dollar Wreck. The episodes air on Monday nights at 10 p.m. and chronicle his latest attempt to discover the treasure he is sure awaits him aboard the Republic.

The RMS Republic, a 585-ocean liner and sister ship of the Titanic, sank south of Nantucket in 1909.

A boisterous man with an eye patch and booming laugh, Mr. Bayerle has now been searching for the Republic in some capacity for over 35 years. He located the ship in 1981 after two and a half years of preparatory research. The ship, a 585-foot White Star ocean liner and precursor to the Titanic, went down on Jan. 23, 1909 after being struck by the SS Florida, an Italian liner, on a particularly foggy day. Six passengers died and no cargo was recovered. The wreck lay 50 miles south of Nantucket.

In 1978, Mr. Bayerle, then living in New York, received a hefty settlement in an anti-trust litigation suit and considered his options. “I could open a dive resort in the Caribbean or I could look for this big shipwreck with rumored fabulous treasure,” he told the Gazette by phone. He picked the treasure, and decided to make the Vineyard his base, buying an Oak Bluffs home from Vineyard contractor Paul Adler and opening a dive shop in Vineyard Haven across from the Black Dog Tavern. Though Nantucket was closer to the wreck, the Vineyard was better situated to receive supplies from the mainland.

Mr. Bayerle taught many Vineyard residents how to scuba dive as well as piqued people’s interest in treasure hunting, including underwater explorer Barry Clifford.

“Barry asked what I was doing on the Vineyard.” Mr. Bayerle recalled. “I said, I’m going treasure hunting.”

He remembers his time on the Vineyard fondly, and not just for the adventure.

“The happiest days of my life were living with my little family in my little house in Oak Bluffs on the Lagoon,” he said. But he could not ignore the siren call of the Republic’s mysterious treasure.

Mr. Bayerle says the Republic’s cargo holds a cache of 150,000 American Gold Eagle coins that would amount to $1.6 billion in today’s value. He maintains the treasure was a secret loan for the Russian tsar and its loss was covered up to keep stability in Europe. His research into the RMS Republic is documented in his 2013 book, The Tsar’s Treasure.

“It will be one of the greatest treasure recoveries of all time, if not the greatest,” he said.

In the book, he delves into international politics, internal banking documents and conspiracies. The depth of his research was one of the main reasons the History Channel had interest in the show, he said.

Paul Adler's footage is featured in Billion Dollar Wreck on History Channel.

“I’ve proven the cargo with the research. Now we have to validate the proof with the recovery of the cargo,” he said.

The sixth episode of Billion Dollar Wreck, which aired Monday, featured archival footage of the treasure hunters loading supplies at the Steamship Authority shot by Mr. Adler, who joined Mr. Bayerle on his second expedition to the wreck site in 1983. More of Mr. Adler’s footage will be shown next Monday.

Mr. Adler remembers capturing the moment when the sonar detected the ship, the first time it was verified by equipment, he said.

“You can see all the needles jump when we went over the ship,” he told the Gazette. There is also a lot of footage of sharks as divers enter the water. Mr. Adler, though a certified diver, remained firmly on the boat.

In the series on the History Channel, Mr. Bayerle has returned to the water with the aid of his estranged son, Grant Bayerle, who now handles the financial and operational aspects of the salvage operation. Whether they found treasure won’t be revealed until the final episode.

“I came into this project thinking it would be three to five year project, and now, here 35 years later, I’m still doing it,” Mr. Bayerle said. “Be careful what you wish for.”

Billion Dollar Wreck airs Monday nights at 10 p.m. on the History Channel. Episode of seven of the eight-part series is on March 21. Past episodes can be viewed at history.com.