Any exercise in futurism has to start with the current and familiar. As McLuhan observed, "We look at the present through a rearview mirror and march backwards into the future." Futurists sometimes distinguish between the probable future, which will develop from what we already know and have experienced, and the imponderable future, which will come about as the result of events, inventions, and social currents that are difficult to predict except in the very near term. We must also be cognizant of what has been termed "black swans," those outlier events that are completely unforeseen or considered very unlikely, but that cause a disproportionate degree of disruption if they do occur. Example might include the financial crisis in Asia that followed the devaluation of the Thai bhat in 1997 and the chaotic effect that the eruption of the Iceland volcano Eyjafjallajokull had on European air travel in December 2009.