Holes

From Zelda Wiki.org

These holes in the ground can be found on many of the islands in the Great Sea of The Wind Waker. Sometimes they are hidden beneath rocks or are hard to find. When Link falls down a hole, he screams. The holes may lead to areas containing various bonuses, and some must be entered during the course of the quest.

Holes also appear in The Legend of Zelda: Twilight Princess and usually contain monsters and treasure of some kind.