Uwe Kersken

They say it's over 2000 years old, and more than 4,000 miles long. But even today nobody really knows for sure. The Great Wall of China is one of the world's famous buildings but it is still the least known. British writer and historian William Lindesay has lived in China for twenty years. Exploring the Great Wall has become his lifetime obsession.

Oceanographers have been gripped by a new spirit of discovery and have undertaken the biggest population census of ocean species ever conducted - a "Census of Marine Life". The quest: to find out when and where it all began. Where did the water come from? How was life created in the oceans? And how did it evolve to the enormous diversity we see today? Join National Geographic as we travel more than 4 billion years into the past to uncover how oceans and marine life came to exist.