On the island of São Vicente in Cape Verde, the community of Portelinha looks from a distance like any other suburb of Mindelo; up close, the logos of the major oil corporations become visible and abundant on the walls of its scattered houses. Stretching across hills and open fields, these are known as Casa Tambor: self-built houses made from discarded and flattened metal oil drums, most of which were originally filled with crude oil in west African countries and then traded on.