The term ROCK, HOLLOW IN can be solved by three crossword solutions: CAVE (4 letters), CAVITY (6 letters), and CAVERN (6 letters). A CAVE is a natural underground space large enough for a human to enter. It is formed by the weathering of rock and often extends deep underground. A CAVITY is a small hollow or hole in a solid object, such as a rock. A CAVERN is a large CAVE, usually with spectacular formations of stalactites and stalagmites.