Secret Grotto
From Zelda Wiki.org
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.

