Possible Clues:

• Concave button
• Lint-collecting bellybutton
• Feature of many a stomach
• Concave object of reflection?
• Common type of bellybutton
• Belly button, for 90% of us
• Roughly nine of 10 people have one
• What many a navel-gazer gazes at
• Navel unit?
• Bellybutton variety