Sea Monsters is a 2003 BBC television trilogy which used computer-generated imagery to show past life in Earth’s seas. In the U.S. it was known as Chased by Sea Monsters. It was made by Impossible Pictures, the creators of Walking with Dinosaurs, Walking with Beasts and Walking with Monsters. In the series, the British wildlife presenter Nigel Marven is shown travelling to seven past seas in the history of the Earth and scuba diving there, in order of dangerousness with the most dangerous last.

He travels in a white sailboat or motorboat roughly 24 m (80 ft) long named The Ancient Mariner. His time-travelling device is not mentioned or shown, and the closest thing to it is his time map, showing the timeline of the seven deadliest seas and the creatures that lived at the time. He uses a scuba set with a fullface mask so he can talk underwater to produce the commentary. He performs some dives using a strong shark cage, which is spherical to make it harder for large sea creatures to bite it.