The Fifth Elephant (legend)
From Discworld & Terry Pratchett Wiki
On the back of Great A'Tuin, supporting the Disc, stand four elephants. There is a legend that in the early days, a fifth elephant stood on A'Tuin's back, but fell off. After a while it smashed into the surface of the Disc, depositing huge amounts of fat, treacle, gold and other resources.
This story, and the metaphors derived from it, provide the title The Fifth Elephant for the novel about valuable deposits of eldritch elephant fat in Überwald, things which do not exist, and things which are not as they seem.
The Fifth Elephant is a pune, or play on words, based on the "fifth element" of Roundworld cosmogony from classical Greek times to the mediæval period. It was theorised that terrestrial matter was made of the four elements earth, air, fire and water, but that celestial bodies were made of quintessence, a fifth element. Alchemists believed that small amounts of quintessence were also found terrestrially.