There is nothing quite so frightening as the idea of a sea monster. Unlike T. rex and other giant dinosaurs, which went extinct, might sea monsters live on? Might they lurk beneath the leaden cloak of the oceans, breaching occasionally into view? Through the ages, serious mariners have returned to port with accounts of huge, snaky beasts baring teeth and trailing feathery manes, undulating through the waves or rearing like a horse. Stories about water serpents have slithered into many cultures. Three legends—from Scotland, North America, and China—are plumbed in these pages. But what about the science of sea monsters? In fact, there was a time when they did exist. About 250 million years ago Earth's continents were gathered into one landmass, Pangaea. Shallow seas and the lack of significant marine predators created new niches for many reptiles that had developed on land. They wriggled into the water, swam, reproduced, and died, becoming the fossils on which the computer-generated art in this article is based. They remain, with good reason, the stuff of nightmares.