In Laguna Niguel, California, on Saturday, about 70 people dropped their pants and mooned Amtrak and Metrolink trains during the 33rd annual 'Moon Amtrak' event.
People of different generations lined the tracks to continue a traditon that began in 1979, when a man at Mugs Away Saloon dared his friend to moon a passing train in exchange for a free drink.
Now, veterans return year after year to drop their pants at unwary passengers.
According to the Laguna Niguel Dispatch, the event used to attract around 10,000 people to moon the trains.
This year, a no-parking zone stopped participants from parking in the street, and police patrolled the area, which may account for lower numbers.
Some train passengers have been known to moon the mooners in return.