Compressed at the time of corking, the natural cork stopper is impervious to liquids but porous enough for air to slowly enter the bottle. When wine bottles are stored lying down, the end of the cork is fully submerged; this is crucial for the aging of the wine, as it allows the necessary gas exchange. This mixing of the wine with oxygen generates its slow oxidation, developing its maturity and complexity. The wine’s acidity thus decreases and the tannins melt.