Cards under another card count as in play, but they have no title, keywords, HP, game text, etc. They only retain their type (Hero, Villain, or Environment), and which deck they belong to. This is also true of face-down cards. When a card is destroyed or leaves play in any other way, any cards under that card are destroyed, going to their appropriate trashes. Cards under a card in play cannot be affected by anything other than the card they are under, or an effect that specifically targets cards under other cards. You can look at cards under another card at any time. You cannot look at face-down cards, unless an effect says otherwise.