I would say trading. China has the largest population of any other country in the world, over 1 billion people live there. I doubt there's anywhere someone hasn't explored already. Plus, with more than 1 billion people there, all the land is needed, either for living on, or using for farming. Live stock could also be a option, but America's livestock system is pretty set in place. I don't think we need anymore animals imported from other countries. Trading is the only viable option left. A lot of our trading is with China. It is not uncommon to look at something you own and then see the MADE IN CHINA on it.