23 The global cargo ship network a b 0 5000 10,000 15,000 20,000 distance (in km) 1 10 10 2 10 3 10 4 10 5 number of journeys observed gravity model random traffic observed journeys N$_{ij}$ 0 1 10 100 1000 predicted journeys F ij 0 1 10 10 2 10 3 1 10 10  2 10 3 10 4 number of links FIG. 6: (a) Histogram of port-to-port distances travelled in the GCSN (navigable distances around continents as indicated in Fig. 1). We overlay the predictions of two different models. The gravity model (red), based on information about distances between ports and total port calls, gives a much better fit than a simpler model (blue) which only fixes the total number of journeys. (b) Count of port pairs with N$_{ij}$ observed and F$_{ij}$ predicted journeys. The flows F$_{ij}$ were calculated with the gravity model (rounded to the nearest integer). Some of the worst outliers are highlighted in blue. ◦ : Antwerp to Calais ( N$_{ij}$ = 0 vs. F$_{ij}$ = 200). △ : Hook of Holland to Europoort (16 vs. 1895). ⋄ : Calais to Dover (4392 vs. 443). □ : Harwich to Hook of Holland (644 vs. 0).