Walker picked up U-99 on her sonar and attacked with depth charges , forcing the submarine to the surface .
Shortly afterwards , U-99 , which was trying to slip out of the convoy on the surface , spotted Walker and dived .
Walker picked up U-99 on U-99 's sonar and attacked with depth charges , forcing the submarine to the surface .
Shortly afterwards , U-99 , which was trying to slip out of the convoy on the surface , spotted Walker and dived .