Volcano (Lethal Lava Land)
It is more than likely self created. It is also in the center of the level, which backs up the latter.
Every few seconds, the volcano will erupt a barrage of little bits of fire. These flames will linger for a brief moment. Whenever the volcano erupts, the spinning platform surrounding it will stop, and after the eruption it will continue to spin. This makes it hard to enter the volcano.
Much like the Igloo, it appears as though nothing can fit inside (even if so, whatever entered would be incinerated), but alas, entrance is available. The inside is filled with lava and platforms that follow along a stream of black orbs. Getting to the top is difficult, even though there aren't any enemies apart from several Podoboos and Pompeiis.