The conventional process for the production and use of materials has been to extract natural resources such as fossil fuel or minerals, refine them, modify them further through manufacturing, and then distribute them as products. At the end of their useful lives, the products are discarded, usually to a landfill or an incinerator. Based on current material practices, society faces a number of problems, including the dispersion of persistent toxics and other contaminants throughout the Earth, depletion of nonrenewable resources such as petroleum, destruction of biodiversity, and the depletion and contamination of aquifers and drinking water sources. There is a growing perception that these material practices are unsustainable.