Quicksand Pits were found on Plunder Island.
They were accompanied by a plaque:
"Quicksand Pit. Quicksand pits of this type are common throughout Plunder Island's nature trails. Many an unwary traveler has found himself trapped and unable to escape."
When Guybrush Threepwood found himself stuck in one, he was able to escape by getting hold of a life-saving vine.