This Spanish winemaker's cellar is situated 25km from Bilbao – and 20m under the Bay of Plentzia.

Aged for up to 18 months on the sea bed, Crusoe Treasure's reds and whites are made with Tempranillo and other classic Spanish grape varieties. Under the increased atmospheric pressure, darkness, and even temperature of their marine environment, they turn unusually soft and silky, says the firm's founder Borja Saracho.

The method is inspired by tales of Champagne rescued from shipwrecks after hundreds of years and praised for its unique characteristics by those who have drunk it.

The Spanish government granted the firm 500 square metres of sea bed to age thousands of bottles in cages within concrete structures that also act as an artificial reef to help sustain marine wildlife.

"By placing the winery there what we create is a protected space where many species can hide and build their new home.

"For us it is very important that we are not only producing underwater wine but that it's a wine that somehow helps the sea and the ecosystem," said Anna Riera, marine biologist and Crusoe Treasure Winery communications manager.