Thick, bubbling mud, warmed by geothermal heat. If you were in a spa, it would be enjoyable, but currently, you're waist-deep in the stuff, after falling into a pit of it at the bottom of a chasm. You can still move, just barely, and so you begin to trudge slowly through the nigh-immovable mud, going for a hanging vine to try and climb back up.

It's only then you feel something else in the deep mud with you, many of them, moving swiftly and with ease, and all coming right towards you...