Kenneth Branagh - Narrator: Three hundred million years ago, much of the Earth was covered in vast swamp forests. Here nothing is as it seems.
Kenneth Branagh - Narrator: Even the air is alien. Over a third is oxygen, much higher than at any time in Earth's history. This oxygen-rich atmosphere has fueled the growth of new, super-sized arthropod predators.
Kenneth Branagh - Narrator: There are other dangers in the swamp apart from predators: the water levels can change rapidly.
Kenneth Branagh - Narrator: The rising water levels suit one group of hunters: amphibians. They've continued thriving over the last sixty million years. Their thin skin still restricts them to the water's edge, but they're now powerful predators, with a devastating pair of jaws, ready to ambush anything that wanders within reach.