Millennium Prize Problems
Mathematicians want to know when a certain function, called the Riemann Zeta function, written ζ(s), equals zero. There are many well known values of s where ζ(s) is zero, composing the negative even integers. The Riemann Hypothesis says that apart from these, ζ(s) equals zero only when s is a complex number with real part 1/2 or a negative whole number. As to whether this hypothetical is one of pure or applied math, a particular characterization of the machine to measure earthquakes gives real-part-1/2 spikes.