Salty Atlantic water and wildlife on the ocean floor give the wines a unique flavour, says this winery.
Dive down into the waters off northern Spain and you may stumble upon some unexpected treasure.
In the Bay of Biscay, a Spanish winery is ageing its bottles at the bottom of the ocean. It says the salty waters and wildlife on the sea floor give the wines a unique flavour.
This pioneering idea is the brainchild of Borja Saracho, founder of Crusoe Treasure Underwater Winery.
He began the deep-sea project after noticing that bottles of wine recovered from shipwrecks sold for high prices at auction.
In 2010, wanting to explore this area further, Saracho began researching if the ocean could change wine for the better. And the results, he says, were exciting.
"We saw that every three months when the experts come to Plentzia, they saw the same story, they preferred the sunken sample," Saracho told Associated Press reporters, following tests of his water-borne wine with those aged on dry land.
"It was very curious," he said. "[Aged under water] the colour changed a little, the red wine turns to a more violet colour and the white wine turns to a more green colour. And in the nose, there were quite different aromas and in the mouth [they] were really soft.
"Everybody was very surprised about the evolution of the different grapes," Saracho added.
This trail-blazing research became Crusoe Treasure, a fully-fledged commercial venture.
Their underwater winery is in the Bay of Plentzia, just off Spain's north coast, in a cool, dark area of the Atlantic Ocean.
At the facility, white wines are aged for around six months at sea, while red wines typically spend 12-15 months under the waves. Crusoe Treasure has so far produced ten different wines.
Analysis by Excell Iberica, a wine research laboratory, has shown they are chemically different to wines aged on land.
"You can think that we are crazy, and all this is marketing," says Saracho. "But after hundreds or thousands of analytics, we have seen the different evolution of the chemical components."
He pointed to several factors affecting the ageing process under the ocean, such as pressure and temperature change, less gravity and constant movement.
Water flowing from the nearby Plentzia River helps maintain temperatures around the cages at between 12 and 17 degrees Celsius. Meanwhile, the current around the winery is rough, so clean water constantly washes over the bottles.
However, although these may be ideal conditions for the ageing process, extreme weather is always a risk.
Saracho recalled one "crazy storm" which gave him and his wife a few nail-biting days.
"I drove here and I was praying," he said. "I believe in God and I was saying, please help us. Because I thought that with the crazy waves there, the facility will be appearing on the beach."
After one week, when the visibility improved, Saracho dived and found that, although there had been some "movement", the facility was "quite safe."
The underwater cages are not just a treat for wine lovers, marine life adores them too.
Crusoe Treasure's facility is 20 metres underwater and has been designed to act as an artificial reef.
The winery employs a marine biologist to periodically monitor biodiversity around the facility, which has become a haven for more than 100 species of flora and fauna.
"Now we have a facility to store 25,000 different bottles of wine," says Saracho. "We are very proud of that, not only because of the wine evolution but because the marine life is living there."
He continues: "Every time that we dive there, it's amazing because we are surrounded by hundreds of different fish. We see octopus, rays, seahorses, squids, big fish with local names, and sometimes dolphins. It's really nice."
The winery also offers tours for wine-loving tourists, particularly from Europe and the United States.
Many holidaymakers, who enjoy visiting wineries, relish the rare opportunity to see one underwater. After a cruise over the bay, they can sample some of the wines, too.
"The taste is just absolutely unique. It's not sweet, but at the same time, it's not bitter. It's difficult to explain. You have to try it to really know," said tourist Alan, who was visiting with his family from Miami, US.
When visibility is good, guests can see the barnacle-encrusted cages and bottles through a camera. If that is not possible, monitors show scans of the seabed, alongside the location of the winery beneath the boat.
"You will never tour another vineyard underwater. You'll never be able to go out on a boat ride at a winery. It's just unique. I mean, tell me where else you get to do this?" said Alan.
Crusoe Treasure claims to be the only underwater winery in the world.
Their facility currently holds some 2,000 bottles of Syrah, 3,000 bottles of White Grenache and 480 bottles of Viognier.
It will submerge more over the coming months, which the winery hope will develop their unique, maritime flavour.