The heat can theoretically be harnessed to generate electricity. Oxen and horses were sometimes harnessed together. Energy has been harnessed by human beings for millennia. Geothermal energy can be harnessed to for electricity generation and for heating. Green energy includes natural energetic processes that can be harnessed with little pollution. Several rivers have been harnessed for hydroelectric power. The energy source harnessed to turn the generator varies widely. The often relentless bora wind is also being harnessed for power generation. Geothermal energy is a product of volcanic activity that the Philippines has harnessed more successfully. Historically, humans have harnessed biomass-derived energy since the time when people began burning wood fuel. Oxen are thought to have first been harnessed and put to work around 4000 BC. Rivers were harnessed to power grain mills and sawmills, and the numerous harbors facilitated trade. In some cases the water power has been harnessed to drive underground mills. In practice, only some of this energy could be harnessed. Manufacturing activities were established that harnessed the Magog River's hydropower.