WILD RACES
Susitna 100 
Here's an article about "the race across frozen Alaska, including lakes, swamps, and rivers.  How would you like dragging a sled along behind you as you run?  What happens when the ice starts to melt...? 

The Canadian Death Race
Their slogan is "It's a Killer!"  Runners are advised to "prepare for the worst but hope for the best."  Lovely.  And guess what?  There's even a "Kids' Death Race" for ages 16 and under, a 5K "mini-extreme" run including a mud pit and a big hill.  Go ahead, try asking:  "Hey Mom, may I run the Kids Death Race?"

Antarctica Marathon and Half Marathon
Across glaciers, past the research bases of several countries.  It will only cost you about $5,000 to do it, not including the cost of thawing you out after the race.  Wear your best tuxedo, the penguins do!  That's the easy stuff, near the coast.  For a related adventure, there is the Ice Marathon in the interior of Antarctica!  Brrrrrr.

North Pole Marathon
Speaking of cold running, how about putting on snowshoes and doing eight laps on a sheet of 6-12 inch thick ice over the Arctic Ocean?  At 15 below zero.  For fun.  The pace might pick up if a polar bear decides to join in!  Some runners wear goggles with little fans that defrost them.  Go ahead, sign up.  The registration fee is only $10,000.

Don't Get Lost Adventure Running
The name kinda says it all, don't you think?  You are required to carry a first aid kit, a whistle, and an emergency blanket.  Oh, and don't forget the map! 

Andes Adventures
How about running on the Inca Trail to Machu Picchu?  The Andes Mountains in this area of Peru are at an altitude of 11,000 feet and higher.  You might want to bring an oxygen tank.

Marathon des Sables
"The toughest footrace on Earth," with 600 competitors from 30 countries, all of whom are crazy enough to spend seven days running 150 miles across the Sahara Desert in Morocco while carrying gear.  Even the camels think this is silly...   

Australian Three Peaks Race
Ahoy, mate!  You've heard of triathelons involving running, swimming, and biking.  Well, how about a race involving three non-stop days of mountain running and sailing?!  This gives a whole new meaning to "ship shape."