What material is NOT suitable for underground fuel supply lines?

Cast iron is not suitable for underground fuel supply lines primarily due to its susceptibility to corrosion and brittleness over time. While cast iron was historically used in various applications, it does not provide the durability and resistance to environmental factors that are necessary for underground installations.

In contrast, both listed non-metallic and listed metallic materials are designed and tested to meet specific standards for underground use, ensuring longevity and safety. Non-metallic options often offer corrosion resistance, while certain metallic materials like stainless steel are known for their robustness and resistance to rust and other forms of deterioration. Therefore, the selection of materials for underground fuel lines must prioritize those that can withstand the conditions they will encounter, making cast iron a poor choice.