Air cargo is the method by which most of the world's art travels. This is because air cargo provides the fastest, most secure, and most trackable means to get artworks to their destination. The most significant paradigm shift in the global transportation system occurred in the 1970s with the introduction of the shipping container. The key feature of the container is that it is of a uniform size. Upon arrival in these ports, containers are off-loaded, customs-cleared, and placed on road or rail connections to continue the freight on to its destination. Local transport, or as it can be frequently referred to in the transport industry, "The Last Mile", can be the most challenging and expensive part of the art shipping process. Airport supervision is provided by firms that also provide security services for high-value deliveries. The most frequent reason for using a courier is when a museum sends one of its star artworks for a temporary exhibition.