A bottle of Champagne typically contains approximately six bar of pressure.
The pressure is created by carbon dioxide produced during the wine’s second fermentation in the sealed bottle. Yeast converts added sugar into alcohol and carbon dioxide; because the bottle is closed, the gas dissolves into the wine. At roughly six bar, Champagne has considerably more pressure than many lightly sparkling wines.
That pressure explains why Champagne bottles use thick glass and specially shaped closures. The cork is held in place by a wire cage called a muselet, and the bottle is opened carefully because the cork can leave the neck at high speed. Chilling the bottle reduces the energy of the escaping gas and helps limit foaming.
Pressure varies with temperature and production conditions, so six bar is an approximate figure rather than an identical reading for every bottle. The familiar “pop” is a release of dissolved carbon dioxide and compressed gas.