The students would emerge on their bikes, in the tens of thousands, seemingly out of nowhere. Like a flash mob on wheels, they rode for hours in the night, by the light of streetlamps, sometimes bursting into verses of the Chinese national anthem. Some carried Chinese flags.

They were making the 40-mile journey from the Chinese city of Zhengzhou, the capital of Henan Province, to the neighboring city of Kaifeng, a cycling trip that can take up to five hours one-way. Why? For the sake of it.

It isn’t clear when the craze really took off, but by early this month, it was drawing hundreds of riders seemingly every night. At first it had been celebrated online and even by the People’s Daily, the Communist Party mouthpiece, which described the night rides as “a symbol of youthful energy and the joy of shared experiences.”

But over the weekend, after the number of riders swelled to the tens of thousands in a country where officials are increasingly wary of impromptu crowds, local authorities shut it down, citing traffic and safety concerns.

The nighttime escapades started in June when four college friends in Zhengzhou decided to bike to Kaifeng to eat a type of soup dumpling that is the city’s specialty, Chinese news media has reported. Soon, hordes of college students were making the trip, too, helped by easy access to shared bicycles that are cheap and ubiquitous in many Chinese cities.