Jonah being cast overboard to a sea monster in Abraham Ortelius's Theatrum orbis terrarum (Theater of the World) of 1595. (British Library)

In this 1367 nautical chart by Francesco and Domenico Pizzigano, an octopus drags one unlucky sailor off a ship while a dragon flies off with another. In this case, Van Duzer thinks the cartographers believed they were warning of a real danger to mariners. (Biblioteca Palatina)

An ichythyocentaur (part man, part horse, part fish) plays a viol while swimming along in Abraham Ortelius's 1571 Theatrum orbis terrarum. (British Library)

Sea monsters attack a ship off the coast of India in Abraham Ortelius's 1571 Theatrum orbis terrarum. (British Library)

Whales smokestack-like blowholes attack a ship in Olaus Magnus's Carta Marina. This map is an absolute treasure for sea monster aficionados. Magnus made the original version in black and white in 1539. This image and the one that follows come from a color version created in 1572. (Sveriges nationalbibliotek)

A walrus-like creature hauls out on the coast of what's now Finland in Olaus Magnus's Carta Marina. (Sveriges nationalbibliotek)

Nothing puts a smile on a sea monster's face like a naked lady in a seashell. The lady here represents Bold Fortune, and she appears in a map made in 1565 by Paolo Forlani. (Library of Congress)

This aquatic unicorn adorns a map in Cornelis de Jode's Speculum Orbis Terrae, published in 1593 (British Library)

This fish-bird hybrid appears in Cornelis de Jode's Speculum Orbis Terrae, published in 1593 (British Library)

This spiky sea elephant appears in a 1572 map by Tommaso Porcacchi. (British Library)

The flying turtle above comes from a map of northern Europe published in 1558. (British Library)

This turtle-like creature appears in a 1578 map by Gerardus Mercator, who invented the projection that bears his name. (British Library)

The polyp, or octopus, depicted here looks more like a lobster-fish hybrid. It appears in Hortus sanitatis, an illustrated encyclopedia published in 1491. Books like this were often an inspiration for cartographers. Van Duzer's book includes an illustration of a similar creature clutching a sailor in its claw. (British Library)

Abraham Ortelius's 1598 map of Iceland boasts an impressive menagerie of sea monsters, several of which are visible in this detail. (British Library)

Whales and large fish-like creatures being mistaken for islands is a repeated theme in medieval manuscripts and maps. In this early 13th century bestiary, seafarers land and light a fire on the beast's back. In many accounts this ends badly for the humans. (British Library)

Two sirens hold mirrors and comb their hair -- presumably to more effectively lure sailors to their doom -- in Diego Gutiérrez's 1562 map of the Americas. (Library of Congress)

It's good to be the king. Here King Manuel of Portugal rides a sea monster off the southern tip of Africa in Martin Waldseemüller's 1516 Carta marina, a nod to Portugal's maritime dominance at the time. (Library of Congress)