The Boulder Cave, located inside the Volcano, is the source of all the boulders that roll down the sloped ledge whenever a player is present in the Volcano. The Boulders are spherical in shape and emit a red glow. They are fairly large, but still avoidable once the player has detected a familiar pattern within the way that these boulders flow. They are very slow making them easier to dodge.
The Cave itself is located at the top of the obstacle. Once entered, the cave makes a sharp left. The cave narrows to an almost player-sized wall (Boulders will spawn halfway inside the cave).
Unlike in the actual volcano course, you will not take damage when you get past the mouth of the cave. Furthermore, boulders will not spawn when you're in the back end of the cave.
In the top-right corner, there will be a small gap in the ceiling in which light will pour out of. If the player looks out of this hole, he/she can see a large, underground grove of Oak and Birch trees, which is the Underground Grove. You need 2+ more players in order to get through that hole, and to find out how, click the link. That grove can also access the Green Box.