With eight major crude oil pipelines terminating there, Cushing, Oklahoma is the largest hub for the distribution of crude oil in North America. The Cushing pipeline interconnect is spread over nine square miles and has crude oil storage capacity of more than 33 million barrels. The crude is stored in tanks that are spaced far apart to minimize the possibility that a tank which catches fire will ignite other tanks. In earlier days, when demand exceeded pipeline capacity, railroad tank cars were used to transport the additional oil. Dikes are built around the vessels to contain any fluids from a storage tank that begins to leak due to corrosion, lightning, or other damage. These dikes, called firewalls, prevent those fluids from escaping and polluting the surrounding area.