Image caption Up to 10,000 people show up each year to bare their behinds to passing Amtrak trains

The southern Californian city of Laguna Niguel has been enjoying an annual ritual, in which locals and visitors bare their bottoms at passing trains.

For 30 years, the city has hosted "Mooning Amtrak" as crowds line up along the railway tracks, dropping their trousers when a train passes by.

Up to 10,000 people take part, and visitors are encouraged to leave their cars at home and arrive by train.

Local legend has it the tradition began in 1979, after a bar room bet.

Can I decorate my butt? Yes, that's OK Moon Amtrak website

A drinker at the Mugs Away Saloon, which stands directly across the road from the railway, offered to buy a drink for anyone who would run outside and moon at the next train.

One customer took him at his word, and a ritual was born.

Each year the event grows, with crowds swelled by word of mouth and enthusiastic reports from radio DJs on local radio stations.

There were reports in 2008 of surging crowds and drunkenness, and police and a helicopter were called in from neighbouring towns to maintain order.

Last year the city, which lies in Orange County between Los Angeles and San Diego, decided enough was enough.

"Avoid the area this year," the city warned on its website home page. It added in a breezy Twitter feed that the city was "saying 'NO' to crack".

Moon Amtrak enthusiasts were not impressed.

Image caption Each year the event grows, with crowds of up to 10,000 reported

Mugs Away regular Rick Sanchez blamed a "stuffy, yuppie mayor" who had "never even been to a mooning", the Wall Street Journal reported.

The event even has its own website, this year proudly headed "31st Annual Mooning of Amtrak". It also promoted a newer offshoot: "5th Annual Mooning of Metrolink".

It features directions to Camino Capistrano, the road where trousers and dignity are dropped each year, and helpfully lists train times through the day, so that people can schedule their disrobing.

And after 8pm, there is night mooning. "Bring a flash light with plenty of batteries, or better yet, bring a camping lantern," the website advises.

There is even a "Frequently Asked Questions" section for mooning debutantes. "Can I decorate my butt?" is one FAQ.

Yes, that's OK, apparently.