The amazing thing about gold is that it remains unchanged by sea water, even after hundreds of years. But once you’ve discovered it, gold fever seems to infect everyone around it. In 2001, I was excavating a 400-year-old Portuguese trading vessel known as a nao, who’d broken her back on a reef off a remote island off northern Mozambique and snapped in two. Her stern lay nestled in a little valley, 30 metres down, sealed from the water by large stones that had cascaded down on top. She was an important wreck, because very little is known about these early colonial excavation vessels — the shipwrights’ plans were destroyed by fires that swept through Lisbon in 1755, so the only way we can work out how they were built is from remains left on the sea bed.

Once you break open the “time capsule” of a shipwreck and expose the timbers to oxygenated water, nothing stops the decay. It’s a race against the clock to get as much information as possible before it disintegrates. When we lifted the stones off the nao, the timbers underneath were a wonderful Iberian oak, dark and strong, with marks where the carpenters’ adzes had struck. Every few days we lifted sections to the surface and studied them. It was paradise, working on a beautiful wreck in clear waters with a tight team of divers who were passionate about shipwrecks.

We heard a rumour that a local spear-fisherman had found gold near the site. But I was sceptical — the reef was within wading distance of the shore. How could gold have sat unnoticed for some 400 years? We dropped metal detectors onto the sea bed where the bow of the ship had broken off, and the volcanic rock had made huge long tubes down into the sea bed. Then one of the team, Alejandro, put his detector over a hole and instantly it started screeching.

He put his hand in and pulled out loaf-of-bread-sized stones and clouds of sand. His arm went deeper and deeper until, suddenly, there was a pristine ingot of gold sitting on his fingertips.

I was astonished. Underwater, gold is absolutely staggering — the colour leaps out; it’s surreally bright. Even a little fleck sparkles in water. We crowded our masks around his hand, to get as much of an eyeful of this colour as we could. Alejandro carried on digging. The rest of us had nothing to do but sit and watch, but there was an impatience now from the other divers. They were pawing the ground and climbing over stones. Behind their masks they were manic — I’ve never forgotten that look. There was a desperation in the way they were turning over rocks and sweeping away sand. Archeology is about doing things calmly, and this sudden frenzy made my heart sink — I could sense trouble.

Fourteen kilos of ingots came up in the end, but they left me completely cold. In archeology, you look for the human story, in order to peer into the minds of people from the past and their world. This gold was without any artistic value or the imprints of whoever made it or who for. But the power it held over the expedition members was intense.

The island was a dangerous place to be with treasure: people had guns, and if anyone got a sniff of this news it would be easy to find us. We hid the gold under floorboards and tried to clamp everyone’s mouth shut, but the divers wanted to celebrate. Suddenly we had people telling others not to drink in the local bar, in case their tongues loosened. Rifts started to appear; insanity crept in. There were tensions and accusations and people not getting on. When you’re a small, tight team, that’s oppressive.

When the company who were financing us heard about the discovery, things got worse. They sent a guy from the UK to join our team to “help us along”. After a week he admitted he was there to spy on us and make sure we weren’t finding more gold than we were telling them. We were outraged. At least he’d been honest with us, but he kept disappearing to make reports on his satellite phone, so we knew he was watching at all times. And the trip was no longer about uncovering the wreck’s story: we were ordered to search every square foot of the reef crest for gold.

I begged to finish excavating the wreck first. Gold doesn’t corrode in sea water, so it made sense to study the ship’s timbers first. That was not what they wanted to hear. And then we found an ingot had been hidden under nearby rocks in the past few days, and bigger cracks started to show. The wreck was in a delicate state and every day the timbers rotted and lost precious detail.

It was utterly dispiriting. You put your heart and soul into this kind of work, and I’d had such high hopes for the project — but it had turned into a treasure hunt.

Dragon Sea: a True Story of Intrigue, Treasure and Adventure Beneath the Waves (Penguin Paperback, £8.99), by Frank Pope, is out now

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