The Lava Obstacle is a stage in BABFT. It poses a hazard for adventurers, as the lava will constantly burn away blocks. Small rocks also destroy even the toughest blocks. The lava damages your boat when you float on it, especially wood, so avoid wood.
It also has lava coming down the side of the walls of the terrain, but doing no damage. If on the walls you can skip the stage: due to there being that the bottom half of the wall is further out.