Imagine you're a sailor in ancient Greece. You've been sailing for days, got caught in a storm and are hopeless lost. You know that there is a very real chance that you and your shipmates will die at sea if you don't find land soon, but wait just ahead you spot an island. You and your comrades set a course for the "island" and make camp right on the Aspidochelone or the asp-turtle (also known as the Fastitocalon.)
According to Medieval Bestiaries the aspidochelone was a large turtle (sometimes a whale) whose sides looked like beaches. It would rise to the surface of the sea, and entice unwitting sailors to make landfall on its huge shell. As soon as those sailors lit their campfires the creature, feeling the heat, would sink back below the depths, drowning all. The bestiaries seem to draw their descriptions from Pliny the Elder's Natural History although there it is called pristis.
The creature was apparently a very popular myth and often used to represent Satan in moral stories of the time. You will find versions of the aspidochelone in the Physiologus, the old English poem The Whale, The Lengend of Saint Brenden (as Jasconius,) in the first voyage of Sinbad the Sailor, in Milton's Paradise Lost (it is mentioned in the story of the Leviathan [an altogether different creature,]) and by Tolkien in The Adventures of Tom Bombadil.
Look, there is Fastitocalon!
An island good to land upon,
Although 'tis rather bare.
Come, leave the sea! And let us run,
Or dance, or lie down in the sun!
See, gulls are sitting there!
Beware! - Tolkien