Big pieces of limestone. I think that the challenges imposed on pilot who flies in PNW and Alaska are some of the most severe found on the North American Continent. The combination of severe icing, turbulence, mountain flying, engine out procedures in and out of mountainous airports, the insanely rapid changes in conditions at destination airports due to coastal weather patterns, wind conditions in mountainous terrain. Add into it the enormous percentage of the year that these conditions present themselves. Many of these can be found elsewhere in the US including thunderstorms in the plain states, but in my experience the all of these conditions/situations in one place compounded by big rocks make experience obtained in rapid decision making and situational awareness all the more valuable.