U-boot is a Polish and Flemish beer mix made from beer and vodka. It consists of one bottle of beer and one shot of vodka.
It is called a U-boot (German for unterseeboot or "submarine", also known as a U-boat), because a small 40 ml glass of vodka is dropped into the larger half-litre (500 ml) beer glass, and later the vodka glass "surfaces" while the cocktail is drunk and the angle at which the large glass is held decreases from 90° to 0° or less.
U-boot : named after the German for submarine, this Polish cocktail is simply beer with a small glass of vodka sunk to the bottom. Only for the brave.