The Great Wall Marathon, China

Who: 2,500 runners, 60 nations

When: May 16, 2015

Why it's unique: Hitting the wall takes on new meaning in this race that pits you against 5,164 ascending and descending steps of the Great Wall of China before dumping you onto a steady 10-percent incline on gravel and asphalt roads in the surrounding rice fields. Then it's a turnaround back for a second go at the Great Wall…all during the hot and humid season. []