Cave of Darkness is a very dark cave in the Land of Ice. It is where the Ultra Divine Water is kept, guarded by Darkness and his Ice Monsters. The cave is sacred and kept from those not worthy of drinking the water.
Goku entered the cave through a magical pot in Korin Tower and Yajirobe went with him after accidentally falling into the same pot. The two faced multiple obstacles and challenges once inside the cave, but eventually made it to the Ultra Divine Water.
Goku drank the water and suffered a great deal from it, but once he recovered, the water had awakened the necessary sleeping powers within him to be able to defeat the evil King Piccolo.