A deserted island occurs when there is alot of toilet paper in the toilet before you poo (either laid there by you or from the previous user). This enables your poo to stay above the water line causing the entire room to smell much worse than it normally would.
"I went into the washroom to shit and some asshole left a deserted island in the toilet. The smell was awful!"
"The next person that comes in here is going to freak when they see this deserted island I left for them."