In prehistoric times there existed creatures bigger and often fiercer than dinosaurs, whose exploits have inspired more terrifying legends than any other group of animals. Their watery home has kept them hidden from view - until now. Monsters once again recreates the prehistoric world, giving us an insight into the underwater environment of that time and the strange and fascinating creatures that dwelt there. The leading wildlife television presenter, Nigel Marven, explores their world through a series of encounters, where he dives deep into the past and swims with these extraordinary reptiles and mammals. From the fearsome hunter, Liopleurodon, one of the biggest predators that ever existed, to Megalograptus, the most vicious of the spiny sea scorpions, the deadly prehistoric seas are vividly brought to life as we see these creatures through Nigel's eyes. Some of the ancestors of these predators exist today, but even the great white shark and blue whale cannot come close in threat and size to these prehistoric species. reconstruction of these creatures, with details of the modern sites, from New York to Egypt, where water once covered the land.