Aged in European oak sherry casks and new oak casks, the whisky is mellowed in Glenfiddich's unique Solera Vat, a large oak tun inspired by the sherry bodegas of Spain and Portugal. Never emptied, and kept half full of whiskies since 1998, Glenfiddich's Solera Vat is the culmination of curious minds and the pioneering spirit of the family. It's also the secret behind this whisky becoming the first 15 year old expression to enter the top ten best selling single malts.