Which team's 1970s defense was known as the "Steel Curtain"?

Head coach Marty Schottenheimer is most closely associated with which team? Head coach Mike Holmgren is most closely associated with which team? Head coach Jon Gruden is most closely associated with which team? Head coach Tony Dungy is most closely associated with which team? Which team's secondary was nicknamed the "Legion of Boom"? Which team's defensive line was known as the "Purple People Eaters"? Which team's 1970s defense was called the "Orange Crush"? Which team's 1960s defensive line was called the "Fearsome Foursome"?