Prefix 'sub' and 'super'

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Across
  1. 1. A famous superhero
  2. 2. Being better than a human
  3. 3. An underground passage or a type of sandwich
  4. 5. Being of a lower rank
  5. 7. An underwater vehicle
  6. 8. To watch over something
Down
  1. 1. To replace something
  2. 2. Dunking something underwater
  3. 4. A heading that comes after the main heading
  4. 6. Something that a superhero has