Heating oil is oil that is burned as fuel in furnaces inside of buildings. The oil is usually delivered by tanker truck to individual homes and business places, and the oil is stored in oil tanks in the basement or outside, next to the building. If oil leaks into the ground, it's an environmental problem because even a small leak can cause major groundwater pollution, making the water from wells and springs unusable because chemicals from the oil are harmful to both humans and animals. Heating oil is usually dyed to make sure it is not confused with vehicle fuel.