Simplify the following text for a Grade 7 native English-speaking person.
Original: Vonnegut was one of a group of American prisoners of war to survive the attack in an underground slaughterhouse meat locker used by the Germans as an ad hoc detention facility .
Simplified: Vonnegut 's group of American prisoners of war survived the attack . The Germans had kept them in an underground room for storing meat at a slaughterhouse .