Mongabay: After creating a hugely successful science-fiction film about a mega-corporation destroying the indigenous culture of another planet, James Cameron has become a surprisingly noteworthy voice on environmental issues, especially those dealing with the very non-fantastical situation of indigenous cultures fighting exploitation. This week Cameron traveled to Brazil for a three-day visit to the Big Bend (Volta Grande) region of the Xingu River to see the people and rainforests that would …