Fateful error

This fortified city of cobblestone streets was founded by the Portuguese in 1680 and is today a UNESCO World Heritage Site.

Just off the coast, in the estuary that splits Argentina and Uruguay, five orange buoys float above the remains of the Lord Clive, marking the spot where Collado and his team are at work.

Pencil in hand, Collado sketches out a map to show where the 64-gun privateer lies, with a cargo he says also includes thousands of liters of rum, 18th-century weaponry and other vestiges of an era when Europe's colonial powers were battling each other around the world.

Collado, who has done extensive research on the ship and its cargo, says its fatal error that January 6 was casting anchor too close to shore.

The Lord Clive, which belonged to the British East India Company, was sailing at the head of a fleet of 11 ships.

Collado believes its mission was to drop arms and equipment ashore to supply an uprising against the Spanish crown across a vast swath of South America, in hopes of winning control over the region's commerce.

"The ship had incredible firepower," he says. "It's a fabulous historical treasure for research on period weaponry."

But despite its three storeys of cannon, the Lord Clive anchored just 350 meters offshore - making it an easy target for the Spanish, and putting it too close for its own guns, which fired over the city.

"They kept lowering the cannon, but they couldn't hit the city because Colonia sits so low," says Collado.

"The Spanish hit them and hit them, until the ship started to sink."