This engraving from a 1621 historical fantasy shows St. Brendan celebrating Easter mass on the back of a whale. You can read more about the rare book that contains this engraving, and see some more examples of its wonderful seventeenth century fantasy art at BibliOdyssey.
The legend of sailors beaching their ships on the back of a whale in the mistaken belief that they had found land was a popular one throughout the Middle Ages.
Ancient sea legends and Medieval bestiaries claimed that the whale was as big as an island and grew bushes on its backside. It was said that mariners, mistaking the creature for land, would anchor their ships to its side, come ashore, and light fires. The beast, feeling the heat of the fires would plunge into the sea taking hapless crews and ships to their watery deaths. These whale legends became a warning against the wiles, cunning, and traps of the Devil who drags unsuspecting sinners down to Hell. . . .
Source: Tucker, Suzetta. "ChristStory Whale Page." ChristStory
Christian Bestiary. 1998. http://ww2.netnitco.net/users/legend01/whale.htm (14 May. 2005). Image from "Here There Be Dragons: A Medieval Bestiary"
The whale almost always symbolizes the devil. In Book One of Paradise Lost, Milton compares Satan, lying in the lake of fire after being cast down from heaven, to
. . . . that Sea-beast
LEVIATHAN, which God of all his works
Created hugest that swim th' Ocean stream:
Him haply slumbring on the NORWAY foam
The Pilot of some small night-founder'd Skiff,
Deeming some Island, oft, as Sea-men tell,
With fixed Anchor in his skaly rind
Moors by his side under the Lee, while Night
Invests the Sea, and wished Morn delayes . . .