Adventurous zoologist Nigel Marven embarks on an underwater expedition through time to explore the seven deadliest seas in Earth's history, coming face to face with strange and dangerous prehistoric sea monsters.

On a unique underwater voyage spanning millions of years in prehistory, our dauntless presenter explores seven different seas, encountering an extraordinary variety of underwater life from seven different eras. His journey culminates in the late Cretaceous period - the most dangerous period ever in the ocean; the time of the awesome mosasaurs, a 20-metre-long monster with a mouth that could bite a tree in half. Before this period there are many other challenges the explorer must tackle first. In the Triassic period 230 million years ago, he meets and attempts to feed a six-metre nothosaur and the serpent-necked tanystropheus. Further back 400 million years ago and the plants have yet to evolve. The seas are terrorised by giant sea scorpions and even more monstrous, the 12-metre long tentacled orthocones. How does our expert plan to approach these terrible creatures?—Svanis