  It is now commonplace to see the Web as a platform that can harness the
collective abilities of large numbers of people to accomplish tasks with
unprecedented speed, accuracy and scale. To push this idea to its limit, DARPA
launched its Network Challenge, which aimed to "explore the roles the Internet
and social networking play in the timely communication, wide-area
team-building, and urgent mobilization required to solve broad-scope,
time-critical problems." The challenge required teams to provide coordinates of
ten red weather balloons placed at different locations in the continental
United States. This large-scale mobilization required the ability to spread
information about the tasks widely and quickly, and to incentivize individuals
to act. We report on the winning team's strategy, which utilized a novel
recursive incentive mechanism to find all balloons in under nine hours. We
analyze the theoretical properties of the mechanism, and present data about its
performance in the challenge.
