SAN FRANCISCO -- Drivers hustling to the city's bustling North Beach neighborhood through the Broadway Tunnel Tuesday got a pleasant surprise -- etched into the grimy walls lining the roadway was a tranquil urban forest.

The unique mural was created by British-based artist Paul "Moose" Curtis using a technique called reverse graffiti or clean tagging.

With the aid of San Francisco police and public works officials, Curtis spent the night blasting away at the years of grime with a high-powered water cannon.

"It all done by slightly customized patio cleaning equipment," he explained. "The contrast is from where one surface is clean and the other is dirty. It shows just how dirty things are."

Curtis said he came up with his method while working and living in his nature Leeds.

"It was a situation I had when I was a kitchen porter -- cleaning pots in a restaurant," he said. "I remember going into the restaurant one day and seeing a dirty mark and cleaning it off, realizing I had made a bigger clean mark than where the wall was dirty… I ended up having to clean off the whole wall."

"Where I live in Leeds (England) there are a lot of tile walls. You could see where people had rubbed their shoulders when they were on their way home from the bars drunk. I could see the contrast."

But refining his muralist style has not been without his challenges. One week he found himself working with his hometown police, etching an ant-gun message on a wall. The next he was arrested for being a graffiti artist.

"I've had my knuckles wrapped a little because it is still regarded as graffiti," he said. "Authorities don't know how to deal with it."

But Curtis said his work makes people aware of the dangers to their environment.

"People try to ram home things to us about the environment, but nothing beats noticing how dirty a wall is for realizing that," he told KTVU. "People have a 3-second attention span. When they see something like this and realize it's just dirt making the picture, it really sends the point home."

While Curtis talked, he proudly overlooked his work.

"It’s beautiful to have it in a tunnel that has created this pollution," he said "We tried to make the nature look a little fragile, have it bend in the direction of the traffic."

And how long will motorists be able to enjoy his work?

"It can last for a long time," he said. "It will here in a year's time, but just a bit faded."