Not sure if this has been posted before, but this is a really good mud/quicksand scene involving Leslie Carone!:
https://www.youtube.com/watch?v=47bhBiiQ63Q

Good scene...and a bit funny too...as Niven at least twice seems to grab at the young lady's posterior (seeking something to hold on to)...after first pushing her forward (helping her) with a palm to her butt....also funny is how their faces manage to stay completely clean, and, her skirt never once rises up too far...