Nuclear Turtle
When your bowel movement results in a mushroom cloud of stench arising in the immediate vicinity. Then you observe below you in the toilet bowl the illegitimate spawn of some sort of amphibians from the bikini atoll test site. Most of the time a nuclear turtle is of a green color in some or all of its outer coating, although a black and brown variety has been reported.
My sh*t just hit critical mass, nuclear turtle imminent!