Sleepless Waterfall

The Sleepless Waterfall is a location from The Legend of Zelda: Ocarina of Time. Located near the end of Zora's River, it serves as a natural barrier to Zora's Domain. It is impossible to jump through the Sleepless Waterfall, and outsiders can only pass through it by playing "Zelda's Lullaby". Once the melody of the Royal Family of Hyrule is played, the Waterfall splits, allowing passage into Zora's Domain.

Theory
It is unknown how the Zoras pass through the Sleepless Waterfall and into their own Domain, but it is likely that they do not. Instead they probably use the underwater tunnel in Lake Hylia as a means of entering Zora's Domain.