Chicagoans are on the hunt for envelopes of cash hidden at a downtown park by a California real estate investor.
They're following Twitter clues to find the envelopes, which the benefactors say are all in Millennium Park. Some lucky seekers are posting pictures of themselves and their finds on Twitter.
Hidden Cash is a scavenger hunt craze started by Jason Buzi. He says he's just one of several people behind the effort, which he calls a "social experiment for good."
Buzi is encouraging people who find cash to "pay it forward."
The scavenger hunts started in San Francisco and have been spreading to other cities, including New York on Saturday and Chicago on Sunday.
The altruistic group also plans events in London, Paris and Madrid in the next two weeks.