@tomjennings You can extend this to any deep supply chain product: medical, skyscraper, etc. Deep supply chains hide the petroleum magic that makes them work. https://genugreen.files.wordpress.com/2012/07/flow-chart-for-products-from-petroleum-based-feedstocks.png Another: https://pubs.acs.org/doi/full/10.1021/acs.est.7b04573