What does J. mean by "an elephant has suddenly sat down on your chest"?

This phrase is used in Chapter II of Three Men in a Boat. The narrator is discussing the problems involved when camping out, especially when it rains. He paints a detailed scene of the difficulties of putting up a tent in bad weather. When the campers finally have it finished and then retire for the evening, his story takes a bad turn:

There you dream that an elephant has suddenly sat down on your chest, and that the volcano has exploded and thrown you down to the bottom of the sea – the elephant still sleeping peacefully on your bosom. You wake up and grasp the idea that something terrible really has happened.

It turns out that the tent has fallen down around the campers, and the three men are piled in a heap, one on top of the other. The elephant and the volcano are metaphors for something heavy and unexpected landing on you. Because of the possibilities of this happening in real life, J., George, and Harris decide to “sleep out on fine nights” only during this river trip, and to check into the nearest hotel, inn, or pub if it starts to rain. You could call them “fair weather campers.”