Take the DARPA challenge and win $40,000

Hint: It helps if you have a lot of networked, Twitter-savvy friends with camera phones in cities across the country.

On Thursday, mysterious signs began popping up in metropolitan areas nationwide, accompanied by a challenge from the Defense Advanced Research Projects Agency: Be the first person to track down and report the square quick response codes on all the signs across the country and win $40,000.

The signs displaying the codes, also known as QR codes, are to remain in prominent public spaces until 3 p.m. Sunday, Feb. 26. For a chance to win the promised $40,000, prospective participants (sorry federal workers, you're not eligible to play) need only a cellphone and an online social network.

DARPA's unannounced contest, a successor to its similarly themed 2009 nationwide balloon hunt, is intended to study how people use social media during times of crisis.

The contest, which DARPA is calling the Cash for Locating and Identifying QR Codes Quest, will remain open until 12 p.m. on March 8, 11 days after the codes are taken down. At that time, the individual who first submitted all the codes will be declared the winner. If all the codes are not found, then the person who submitted the most will receive a prorated amount of the prize money, according to Jay Schnitzer, director of the Defense Sciences Office at DARPA.

Participants are encouraged to form groups to share code locations with each other and split the prize. The project "seeks to understand how to leverage social media for time-sensitive humanitarian assistance problems," he said.

Federal employees, DARPA contractors and their dependents are prohibited from playing "to avoid the appearance of unfairness and to ensure compliance with all applicable government ethics requirements for federal employees," Schnitzer said.

The winners will be announced the week of March 12.