The Amazon, the world's largest tropical forest and the natural habitat of more than three million species, is under threat as humans continue to tear down and burn whole swathes of the land to make way for cattle ranches, farmland and mines. The so-called "lungs of the Earth," are in danger of reaching a "tipping point," where much of the rainforest turns to savannah, according to experts. Climate scientists say passing that irreversible point would be catastrophic: instead of helping curb climate change, the Amazon would accelerate it. IMAGES AND SOUNDBITES ARRANGED IN SEQUENCES