The Lava Cavern, also known as The Volcano Cavern, Cavern of Lava, or Impossible Cavern, is a tunneling system located in a ledge in the Volcano Obstacle (added in on the Volcano Update).
Although it is an anomaly and a tunneling system, it is still technically a cavern. The cavern is quite narrow and is nearly full of Lava (besides a few rocks that protrude from the walls). The Lava in this cavern produces an immense amount of particles as well as an immense amount of red light, and the obstacle's effects still take place inside the cavern. The Lava and the cavern's walls keep the same coloration and texture material as the rest of the entire Volcano Obstacle. Vehicular access to this is near to impossible.
(Note: It's been confirmed that the cavern is almost like an inner-ring to the Underground Grove, both circling around the volcano for some reason.)
Since its discovery, many people have found particular interest in this anomaly because of what is at the end, the Green Box.
Because of the limited amount of health the player has, the player must act swiftly. There are also a limited amount of rocks in the cavern that are spaced out, which makes it difficult to avoid the Lava.