Abiotic resources are resources that are non-living. These resources fall into the larger category of natural resources, which occur naturally in the environment and are not created or produced by humans or human activity. Human depletion of abiotic resources, such as water, soil, and minerals is a source of concern for humans, as these resources are not easily replenished and are being used above the rate that they can be naturally replaced. Land is one of the most lucrative abiotic resources for humans, as property costs in urban and suburban development has rapidly increased in price and demand throughout cities and communities across the world. Land degradation as a result of extensive use of fertilizers and chemicals as well as development have rendered large portions of land unusable. Along with environmental harm, land degradation is also a large source of lost revenue. Water is an abiotic resource that is of need to all living things. Yet, access to clean and safe water remains a challenge for members of developing nations. Coal remains one of the major sources of heat and electrical energy in the world today, and will likely retain that role for decades to come. That being said, coal depletion is an issue that faces much of the world today, as a major source of energy could soon run out. Oil is also one of the most valuable natural resources to humans, as cars, planes, ships, trucks, and most other forms of transportation require oil. Oil depletion is therefore a major concern, as oil that took millions of years to form from decomposing plant matter is being extracted and burned in a matter of months.