Treasure and shipwrecks go hand in hand – but what are the most magnificent and historically significant items ever salvaged and what do they tell us about the world we live in? The underwater archaeologist Mensun Bound, who was the director of exploration on the team that discovered Ernest Shackleton’s ship Endurance, has written a new maritime history of the world through shipwrecked objects he has encountered over the course of his career.

Here, he reveals some of the most important – and quirky – items that have ever been salvaged from the sea.

A cannon fired in the Battle of Trafalgar

HMS Agamemnon was Lord Nelson’s first major command and his favourite ship, says Bound. “It was the ship on which he first fought the French and it was where he met the love of his life, Lady Hamilton.”

The ship served in the American revolutionary war, the French revolutionary war and the Battle of Trafalgar. “She was behind HMS Victory by about five or six ships.”

The ship later went aground off the coast of Uruguay while chasing a fleet of French ships, and got stuck on a mudbank. “They were able to salvage all the guns except one – in the record, there is a very clear description of how they dropped one in the water by mistake,” says Bound, co-author of the forthcoming book, Wonders in the Deep.

In 1997, he found the missing gun using an acoustic sonar device. “We didn’t really expect to. But it stood out: a cannon in profile.”

After stripping away the iron gun’s crust of corrosion, he made a rare discovery: a number on the cannon matched an archival record of a gun tested and refitted because it had been fired during the Battle of Trafalgar. As such, it is the only known cannon in existence proven to have been fired during the most famous sea battle there has ever been, he says.

“That battle changed the course of history and ended any lingering dreams Napoleon might still have harboured about conquering England.”

A Bible from Endurance

In 1914, Shackleton and his 27-strong crew sailed on the ship Endurance to Antarctica in a bid to become the first men to cross the continent, the last major expedition of what has come to be known as the “heroic age” of Antarctic exploration.

After entering pack ice and encountering gale-force winds, the ship became frozen in the ice and the crew were forced to abandon Endurance and most of their belongings.

“They wanted to try to get to land with the dogs, so they could only carry a tiny number of personal possessions with them: just a couple of pounds in weight,” says Bound, an underwater archaeologist who was the director of exploration on the team that discovered Endurance.

Shackleton tore a few important pages out of his Bible to take with him, such as its inscription from the dowager queen, then left the heavy book behind in the snow.

“But there was a fisherman in the crew called Old McLeod – Thomas McLeod, from Stornoway – who had this Presbyterian devotion to the Bible. We don’t think he could read but he was very religious and he thought leaving the Bible there was tempting fate,” says Bound.

When nobody was looking, McLeod rescued the Bible and hid it among his possessions.

Eventually, McLeod was rescued, along with the Bible. It was later donated to the Royal Geographical Society in London.

Bound still finds it “amazing” that all 28 of the crew survived that shipwreck, and the Bible did too. “It should not have happened,” he says.

An ancient bronze helmet

In 1961, divers discovered a shipwreck off the Tuscan island of Giglio. One of the items salvaged was a Greek helmet, beaten from a single sheet of bronze in about 600BC, when the Etruscan empire was at its peak. “It was made using superb metalwork skills, which we simply couldn’t do ourselves today,” says Bound.

Charging boars are etched on the cheek, and curling up over the eyebrows are open-mouthed, fanged vipers, portrayed in “absolutely exquisite detail”, he says. “This helmet is the finest example of its kind in the world.”

He sees it as an expression of ancient technology. “It was a genuine item of war – whoever owned that helmet, wherever he went in the world, he was sending out a signal that he was important and wealthy and powerful.”

Bound is one of the few experts who has seen the helmet in person. It was discovered by a German diver, who showed it to Bound in the 1980s, a few years before his death.

Since then, Bound says, “nobody knows what’s happened to the helmet”, despite a longstanding campaign for information about its whereabouts by the Italian government.

“It’s one of the big mysteries of archaeology: where is the Giglio helmet?”

Gold from a Portuguese trading ship

In 1554, the Espadarte, a Portuguese carrack, was returning from a voyage to India when it broke its mast and sank near Fort Saint Sebastian on the island of Mozambique.

“The Portuguese were an incredible nation of seafarers, and they lost a lot of ships in the process,” says Bound.

In 2001, he and his team discovered a ship believed to be the Espadarte by combing the seabed of a channel in front of the fort. The treasures they discovered near the wreck included spices (pepper, cloves, nutmeg and mace), cowrie shells (used as money in parts of Africa), 16th-century Ming porcelain – and approximately 50kg of gold.

“I had found gold before, but never in quantities like this – big slab buns of it, which they call bun ingots, as well as broken gold chain and gold jewellery,” says Bound.

There was no evidence that the ship was carrying enslaved people. Instead, Bound thinks the gold on the ship was part of the growing sea trade in spice, silks, ceramics and lacquerware.

“The trade in gold was fundamental to the expansion of international commerce – it was the international currency of the world at that time – and the Portuguese are among the first Europeans to battle their way around Africa. They were trying to find the Silk Road of the sea.”

It is possible it had been cannily exchanged by the Portuguese for silver in the east, where silver was more valuable than gold.

“We don’t know. But the purchasing power of that gold would have been huge. Somebody took a big hit when that ship went down, that’s for sure.”

• Wonders in the Deep, by Mensun Bound and Mark Frary, is published on 26 September