"Sifei is based on friendship, using the internet," says Zhang.

"But we usually don't travel together," he adds alluding to the fact that a large part of the country's ancient architecture is scattered in the countryside or in the wilderness.

Other similar organizations in China often take paid tours on site visits, but he has chosen not to make Sifei just another "travel agency".

"Sifei is a place for us to play together," says Zhang. "It's not about developing a business."

The members, however, do get together a couple times a year, and share their findings and experiences.

And though they keep in touch online most of the time, Zhang says they are close because of a common passion.

"It's important to gain new knowledge on the road," he says. "It's equally important to learn through communicating with different people after returning from a trip."

On the road

Zhang, who was raised in Xingcheng, a small town in Northeast China's Liaoning province, has so far visited more than 1,500 national-level key relic sites, besides lesser-known ones.

Xingcheng was an important fortress during the Ming Dynasty (1368-1644).

And, in 1626, Yuan Chonghuan, a Ming general, successfully defended the fortress while fighting the Manchu army in a fierce battle.

Xingcheng has a well-preserved layout of an ancient city. And Zhang recalls that there was an old temple in his elementary school.

"When I was in class, I often wondered if there was some secret place housing hoards of treasures in that temple," he says.

Zhang's professional background bears no relevance to his passion today.

Explaining how he took up his new calling, the architecture and calendar fan－who attended medical school, and now works at the Chinese Center for Disease Control and Prevention in Beijing－says it all started with a visit to Shanxi province in 2007.

That trip, not only revived his childhood memories but also encouraged him to study ancient Chinese architecture.