A sexual act in which one man takes a pee into a woman's mouth. The woman then proceeds to his anus. She then spits the piss into the butthole, immediately after she clamps the butthole shut. The man then farts or pushes extremely hard to get the traveled piss out and onto the girl's head/face. Like a waterfall...in Mongolia.
Me and my girlfriend tried the mongolian waterfall and we just can't go back! Its great.