"I had heard a bunch of stories about wines ageing at sea," Bruno Lemoine, who runs the cellars of Chateau Larrivet Haut-Brion in the southwest Bordeaux region, explained as he unveiled their findings this week in Paris.

One of the earliest known sea vintages dates from the 18th Century, when the Bordeaux baron Louis-Gaspard d'Estournel sent a shipment of wine to India, whose unsold bottles returned to France mysteriously improved by the hull journey.

The most recent – and extreme – case dates from Friday, when 11 bottles of the world's oldest champagne, salvaged in 2010 from a Baltic Sea shipwreck, were auctioned off in Finland for 109 280 euros ($136 000).

The six bottles of Juglar, four of Veuve Clicquot and one of Heidsieck & Co, were preserved or even improved in the 200 years since the wreck, experts believe, thanks to the ideal conditions found on the chilled, lightless bed of the Baltic.

"I found the whole idea amusing and intriguing," Lemoine said. "So when in 2009 we found ourselves with an exceptional vintage, full of rich tannins, I decided to put it to good use."

"It started out as a lark among friends. One of us came up with the idea and the others ran with it."

First Lemoine asked his barrel-maker chum Pierre-Guillaume Chiberry to build him two small 56-litre wooden barrels in which to age his red wine by an extra six months.

One was to be kept in the chateau cellars, the other sunk underwater among the prized oyster beds of the Bay of Arcachon, north of Bordeaux on the Atlantic coast.

Chiberry set his three top craftsmen to work on the barrels, assembling them simultaneously by hand to ensure they were strictly identical for the purpose of the experiment.

When done – inspired by the challenge – they cycled 150 kilometres to Lemoine's vineyard in June 2011 with the barrels in tow to see them poured full of the 2009 vintage, already aged nearly two years by this point.

It was much better than it should have been

The barrel kept at the chateau was dubbed ‘Tellus’, after the Roman goddess of the land, and the other ‘Neptune’ after the sea god.

"Neptune" was picked up by Lemoine's oyster farmer friend Joel Dupuch and rowed out to the low tide mark, where it was chained inside a concrete chamber that kept it protected while letting the water flow in and out.

"The barrel could roll around a little," as it would if it were lying on the sea bed, Dupuch told the wine lovers and journalists gathered in Paris.

"It was very exposed to the wind and the weather, and it was just opposite the window of my house, so I could keep an eye on it!"

During the very lowest tides it was briefly exposed to the air, around 25 or 30 times over a six-month period.

Both barrels were retrieved in January for the wine to be bottled, tasted and analysed in a laboratory.

‘Tellus’ turned out to be rather disappointing. But ‘Neptune’ was a good surprise all round.

"When we tasted it, it was much better than it should have been," the expert taster Bernard Burtschy told the Paris gathering. At once mellower and more complex than its on-land relative, he said.

Lab tests helped shed some light on the process: despite the barrel's watertight stainless steel plug, they confirmed that the wine was subtly changed by its ocean environment through a process of osmosis.

On the one hand the wine lost some of its alcohol content, while on the other it saw its sodium concentration rise, adding a subtly salty note that brings out the best of the tannins, Burtschy said.

"In ancient times the Romans used to add a little salt water to their wine," he pointed out.

For Lemoine, "Neptune" has yet to tell its full story.

"We've tasted it at a particular point in time," he said. "But you need to see how the wine evolves over a longer period."

He intends to keep checking the vintage over the coming decade -- and already plans new experiments, with different wine-making processes and barrel sizes, to tease out more wine-making secrets from the sea.