Secret Grotto

From Zelda Wiki.org

A secret grotto or simply grotto is a secret cavern or cave found in the world of Hyrule that is sealed either by earth or an enchantment that can sometimes respond to the power of music. The land of Hyrule has a widely unexplored substructure left over from long ago that is made up of cavernous passages as well as smaller caves. It is believed that many of these caves still exist somewhere in the wide world and that they are filled with secrets and treasure just waiting to be discovered.

Some say that the grottos were once used by the members of the Sheikah tribe to hide their supplies and treasure during their ancient service to the royal family. Seeing as the Sheikah have all but vanished since the Unifying War (before Ocarina of Time), the locations of many of the secret grottos have been lost to time. However a skilled adventurer can discover the secret location of many of the elusive caves by using the Stone of Agony. This odd stone has a tendency to rumble when its bearer comes close to a secret cave in Hyrule. It can only be found in Ocarina of Time.

It is believed by some that the Stone of Agony is actually a companion relic of two other items found in Ocarina of Time: the Mask of Truth and the Lens or "Eye" of Truth. And so when all three items are used collectively a traveler can often find his way much easier.