Sunken Treasure (Blue)

Sunken Treasure triggers when it resolves and becomes a defending card on the combat chain. This is an optional effect: You choose whether or not to turn a card in any graveyard face-down at the time the triggered effect resolves. If you turn a card in a graveyard face-down and it is yellow, you will create a Gold token.

The card in the graveyard must be face-up in order for it to be turned face-down. If there are no cards face-up in any graveyard at the time this triggered effect resolves, you cannot do this optional effect.

Unless a rule or effect allows you to do so, you cannot declare defending cards against attacks where the attack-target is an ally you control.