Wikipedia
A U-Boot is a beer cocktail (popular in Germany, Poland, Serbia and Flanders) that is made with a glass of beer and a shot glass of vodka. In Germany, the liquor korn is sometimes used instead, while in Flanders and the Netherlands, jonge jenever is preferred.
It is called a U-Boot (German abbreviation of Unterseeboot, " submarine") because the shot glass of vodka sinks to the bottom of the glass of beer. The shot glass then "surfaces" when the cocktail is drunk.
In Flanders and the Netherlands, it is called a "Duikboot", literally translated "Submarine". It is seen as a variant on the kopstoot (lit. "headbutt") in the Netherlands. The kopstoot is a shot of jenever followed by a beer as a chaser.
In Serbia, it is called "Podmornica", literally translated "Submarine".
In southern Germany, U-Boot is usually served as a combination of Fanta and cognac instead of beer and vodka.
In Mexico, it is called "Submarino", Spanish for "Submarine". Instead of vodka, it uses tequila, and the shot glass must be left inverted inside the beer glass.
In Sorbia, vodka and orange juice is used.