At around 4 p.m. on July 20, Zou Deqiang and his colleague Wang Yunlong were struggling to call a cab. It had been raining heavily all day in Zhengzhou — a sprawling megacity in central China — and few vehicles were on the inundated streets.

The pair, who were in Zhengzhou on a business trip, weren’t overly concerned: Minor flooding incidents happen regularly in the city. They jokingly snapped a video of themselves dipping their feet in the ankle-deep water, and headed for the subway.

It would be the last time Zou ever saw daylight.

Just two hours later, the two men found themselves trapped underground inside a packed subway train amid a devastating flash flood. With help slow to arrive, the passengers became increasingly desperate as the carriages rapidly filled with water. Fourteen people would eventually die in the tunnel — including Zou.

The disaster was just one of many that struck Zhengzhou that day, as surging floodwater left nearly 300 dead across the city. But it was a preventable tragedy.

Over the past two weeks, Sixth Tone has spoken with eyewitnesses, experts, and survivors to reconstruct a timeline of events leading up to the disaster. Our reporting suggests that had swifter action been taken, a fatal accident may have been averted.

By the afternoon of July 20, it was already clear to Zhengzhou’s weather bureau that the rainfall battering the city was no ordinary downpour.

Between July 17 and 20, the city received just under 620 millimeters of rain — nearly an entire year’s worth of rainfall. The bureau issued five storm red alerts — its highest-level alert — in the 24 hours before the disaster in an attempt to raise the alarm.

A red alert should trigger a citywide shutdown of all unnecessary outdoor activities, with students and commuters encouraged to stay at home. But the warnings failed to catch the city’s attention.

From 4 p.m., the rain became even stronger — a record-breaking 200 millimeters of precipitation falling in just one hour. Streets rapidly turned into rivers, and water began pouring into underground parking lots and tunnels.

Yet when Wang and Zou boarded the subway, the city’s metro system was still running an almost normal service. Zhengzhou Metro Group, the network operator, had begun closing some station entrances at 3:40 p.m. to prevent water from flowing onto the platforms. But it did not order a full service suspension until 6:10 p.m.

Wang and Zou ducked into Huanghe Road station and got on a Line 5 train headed west. Line 5 is a busy commuter route that runs in a loop around the city center. That day, there were more than 500 passengers on the train, with many choosing to take the subway as the roads were flooded.

The passengers had no idea at this point that they were in danger. But their train was heading directly toward the most compromised section of the tunnel.

At the northwestern edge of Line 5, there is a rail yard called Wulongkou, from which Line 5 trains enter and exit the main tracks. As the rain became fiercer through the afternoon, the rail yard’s flood defenses began to buckle, sending water cascading down the tunnel toward the nearest station: Shakoulu.

In a statement issued after the disaster, Zhengzhou Metro Group said the rail yard’s flood defenses were destroyed at 6 p.m., leading the firm to shut down the entire network 10 minutes later. But eyewitness accounts suggest damage was apparent much earlier.

By around 5 p.m., floodwater had already breached the wall protecting the tracks inside the rail yard, an eyewitness — who requested anonymity for privacy reasons — told Sixth Tone. A photo taken by the person, time-stamped 5:02 p.m., shows that several parts of the wall had collapsed and disappeared under the water.

Wang Xiaodong, a construction worker who was resting in a porta cabin near the entrance to Wulongkou that afternoon, told Sixth Tone that water began flowing into his cabin at around the same time, forcing him to take shelter on top of a bunk bed. A photo Wang took from inside the room at 5 p.m. shows a man standing ankle-deep in water, with a washbasin floating nearby.

By 5:30 p.m., the flood had also destroyed a brick wall just outside the rail yard’s pumping station, leaving a gap several meters wide, two other construction workers who were near the site at the time told Sixth Tone.

Yet, unaware of the peril that lay ahead, the Line 5 train continued speeding west. At around 5:40 p.m., it stopped at Haitansi station, then restarted and headed toward Shakoulu, the closest station to the rail yard. This was when the passengers’ nightmare began.

Before reaching Shakoulu station, the train came to an abrupt halt, unable to continue due to the torrents of water already pouring into the tunnel. The train was stuck on a steep stretch of track designed to help trains decelerate as they approach the platform.

Video footage circulating online showed subway passengers trapped in a flooded carriage of a Line 5 train in Zhengzhou, Henan province, on Tuesday. Rescue efforts are underway, according to local media. pic.twitter.com/POdT5aBn8a

— Sixth Tone (@SixthTone) July 20, 2021

Floodwater started flowing into the carriages around 6 p.m., according to several survivors. Within minutes, the water was already several inches deep in the rear carriage — where Wang and Zou were located. Zou recorded a video of the partially flooded subway car and sent it to his wife.

As the train rapidly filled with water, passengers sent out desperate pleas for help to family members, emergency services, and via social media. Yet many of them would remain trapped inside the carriages for over three hours, watching the water rise ever higher and struggling to breathe as they gradually ran out of oxygen.

With no signs of immediate rescue, people on the train tried to take matters into their own hands. Staff members managed to open the doors inside the front carriage, allowing those on board to access an emergency walkway running along the tunnel toward the station.