When one craps their pants, and then hides their fouled underwear in the tank of the toilet. A close relative of the Upper Decker.
When Mary was cleaning the bathroom after her party, she noticed her toilet tank cover had been moved. She opened it and found a Sunken Treasure left by John.
When the sphincter has been pushed beyond it's natural limitations and the walls of the anus collapse and fall out; anal prolapse, rectal prolapse.
When my wife gave birth to our third son, I saw her sunken treasure.