Many people struggle to run a mile; others can knock out a 10K on a whim. A few brave souls take on "ultramarathons" — races longer than 26.2 miles (42.2 kilometers). But how far can a human run ...

To answer that question, first we have to define what it actually means to "stop." Dean Karnazes holds the unofficial record for the longest run without sleeping, at 350 miles (563 km), which he ...