It's basically an attempt to get 100 caches in 12 hours.
I managaed to get 67 the first time I tried and 43 the second.
It's crazy fun. All the cachers meet together at 6am and the race is on...
You all follow the same route and see how fast and how many you can find.
The current record I know of is 101 caches in 13hrs.
Check out this for more info on what a cache machine is: