Millions of people spend part of each day running, whether around a track or over the roads or along trails.

However, relatively few take on the challenge of preparing for and running ultra races, those 100-mile treks that are in many ways the ultimate tests of physical and mental endurance.

Watch below as some of the top ultra runners in the world -- including six-time Skyrunner World Series champ Kilian Jornet, seven-time Western States 100 winner Scott Jurek, and three-time Hardrock 100 champion Darcy Piceu -- describe what they consider a spiritual experience, and the bond between runners and nature.