Minecraft wiki entry for Mining, associated text: Dig a quarry down to under Y-level 25, and attempt to unearth a cave. If they did, they can explore that cave. Many of the deeper caves have no surface entrance, and being able to expose a cave that is otherwise sealed to the outside world through a quarry can garner them both large amounts of cobblestone, dirt, and gravel, as well as the rewards of mining out a deep cave. If they don't want to hold on to the stone that is excavated, they can use it to fill in dead ends, allowing them to navigate the remainder of the cave more easily.