At least 12 people died in Zhengzhou, a city of over 12 million on the banks of the Yellow River, in a flooded subway line while more than 500 were pulled to safety, the local government said.

Video on social media on Tuesday showed commuters chest-deep in murky floodwaters on a train in the dark and an underground station turned into a large, churning pool.

“The water reached my chest,” a survivor wrote on social media. “I was really scared, but the most terrifying thing was not the water, but the increasingly diminishing air supply in the carriage.”

Due to the rain, the authorities halted bus services, as the vehicles are powered by electricity, said a Zhengzhou resident surnamed Guo, who spent the night at his office.

“That’s why many people took the subway, and the tragedy happened,” Guo told Reuters.

From the evening of Saturday until late Tuesday, 617.1 millimeters (mm) of rain had drenched Zhengzhou – almost on par with the city’s annual average of 640.8 mm.

The amount of rainfall in Zhengzhou witnessed over the three days was one seen only “once in a thousand years”, local media cited meteorologists as saying.