The modern tools includes several forms of sonar(detection by sound reflection).
sonar
Sonar
If you are meaning to U-boats as in World War two? Than U-boats certainly mean "Underwater Boats"
Submarine
by flying underwater
If you are meaning to U-boats as in World War two? Than U-boats certainly mean "Underwater Boats"
United States.
Most likely Germany, they had the U-boats (underwater boats)
They use a Sonar system.
the chinese people
the chinese people
They where U-boats or Underwater Boats. These ships sunks most likely the most ships in the entire war.