What are some real-life applications of math that make you say, "whoa"?

I would love to hear some cool math applications in the "real world" that people have encountered. Anything that made you open your eyes!

A small but fascinating example is that a friend of mine recently discovered that a question he had in childhood about LEGOs - how to arrange the LEGOs of an old set he had into the largest house possible - was an optimization problem solved with Calculus.

Whether small day-to-day problems like above or more complex examples (say, differential equations in governing biochemical reactions in cells), what are some ways math is applied in the world that open your eyes?