According to the tradition of the Physiologus and medieval bestiaries, the Aspidochelone is a fabled sea creature, often described as a vast sea turtle. It is always described as being so huge that it is often mistaken for an island and appears to be covered in sand dunes, rocky crevices and valleys, and trees and greenery. The name Aspidochelone appears to be a compound word combining Greek aspis - which means either "asp" or "shield", and chelone - “the turtle”. It rises to the surface from the depths of the sea, and entices unwitting sailors to make landfall on its huge shell and then the turtle is able to pull them under the ocean.

There are a few reports of these beasts. The first was by Alexander the Great, where a group of his most trusted men went out to an island that was said by locals to have a hoard of a dead king. The island sank within an hour of the men landing upon it. Sinbad also encountered one on his adventures when he and his crew lit a cooking fire on a small island. Suddenly, the island shook as the monster awoke and retreated under the sea. Sinbad and some of his men didn't make it back to the boats and were dragged under from the pull of the diving beast. Sinbad survived and was later washed ashore a real island. There is also another report of one of these creatures from a group of Irish monks on a sea voyage to America. They wrote that they had encountered a creature that looked like an island but vanished under the waves of the sea.