What does a mammoth smell like? Do dinosaurs bob their heads as they walk, like today's birds? Do aurochs moo like cows? You may soon find out.

From the Siberian permafrost to balmy California, scientists across the globe are working to resurrect all kinds of extinct animals, from ones that just left us to those that have been gone for many thousands of years. Their tools in this hunt are both fossils and cutting-edge genetic technologies. Some of these scientists are driven by sheer curiosity; others view the lost species as a powerful weapon in the fight to save rapidly disappearing ecosystems.

Science journalist Torill Kornfeldt travelled the world to meet the men and women working to bring extinct animals back from the dead. Along the way, she saw a mammoth that has been frozen for 20,000 years, and visited the places where these furry giants once walked. It seems certain that they and other lost species will walk the earth again, but what world will that give us? And is any of this a good idea?