Storage is required in order to meet high seasonable demand for natural gas (i.e. during cold winter months in Europe and the US). Since the system works more efficiently with a constant flow throughout the year, gas is put into storage during lower summer demand periods to be used during peak times. Storage is also used for balancing purposes and/or in times of supply disruption. Gas can also be taken out of storage during peak consumption times, since heating demand for natural gas during winter puts upward pressure on natural gas prices. Natural gas is most commonly held in inventory underground under pressure in three types of facilities: (1) depleted reservoirs in oil and/or gas fields; (2) aquifers and; (3) salt cavern formations. Depleted oil and gas reservoirs are the most commonly used underground storage sites. Pipeline companies rely heavily on underground storage to facilitate load balancing and system supply management. Most storage facilities, such as the Rough Field in the UK are used more for seasonal trading purposes (store gas when it is cheap and sell it when it is more expensive during winter) as well as for technical reasons to maintain the continuous flow in the pipeline. In addition, the European national transmission system can fail during very cold winter days so the Rough Field is maintained to overcome daily shortfalls if such occur