Sushi originates from the practice of preserving fish by fermenting it in rice for months, a tradition which can be traced back over 5000 years. Agricultural rice farming probably began in northern Thailand along the Mekong river and in the surrounding flat lands. In season the fields would always flood, allowing the people to farm fish as well. But the difficult challenge of preserving the bounty for the coming season when the waters receded took centuries of trial and error to solve.