Great Moblin's Keep
Link is able to enter the Great Moblin's Keep after he has obtained the Pegasus Seeds, which allow him to run across the crumbling floor inside of the Keep. On the top floor of the Keep, the Great Moblin will start throwing bombs at Link, after he is defeated the entire Keep will collapse leaving nothing but a wall that will lead to a new cave.
If Link falls down the crumbling floor he will fall down into a new room filled with water. After swimming through the side scrolling area and climbing up the ladder at the end, Link will be lead into a new room with a Treasure Chest containing a Seed Ring.