The 38 Vietnamese passengers trapped inside screamed for help while kicking and punching at the unyielding windows. Finally, someone broke through the glass pane in the front seat where Mung and her 19-year-old son, Tuyen, were sitting.

Eighteen people, including the driver, managed to reach safety before the bus disappeared into the water Monday along with 20 other passengers. Divers located it on the river bottom Wednesday about half a mile downstream from where it was yanked off the road. It’s unclear how many people are entombed inside, but all 20 missing people are presumed dead.

In the past week, parts of central Vietnam have suffered the worst flooding in more than 30 years. At least 45 others have died, besides those in the submerged bus. Churning waters have devoured sections of the country’s main north-south highway the bus was traveling.

When the river started to consume the vehicle, a handful of young men were the first to shimmy out. But their escape was hindered by whitewater-style currents, forcing them to cling to the side of the bus or grab onto nearby power poles. As water began to gush inside, Mung knew it was sinking and they too had to climb out the window.

“We clung to the bus for 15 to 20 minutes. My son told me, ‘Mom, I’m very cold,'” Mung said. “We were together, and he was holding my hand.”

Vietnam, a tropical country mapped by a spider web of rivers, is prone to storms and flooding that kill hundreds of people every year. The central region has been punished especially hard in the past month. The first round of flooding two weeks ago killed 66 people.

Another 4.6 feet of rain has fallen in the past week, submerging more than 220,000 houses and forcing about 173,000 people to flee their homes, authorities say.

Green rice paddies have become endless coffee-colored lakes with only the occasional rooftop poking out of the water. Many flood victims have been stranded without food and water, some forced to wait for help inside attics or perched atop flimsy tin roofs.

In some upstream areas of hardest-hit Ha Tinh province, the flooding is the worst ever recorded with rivers surging to all-time highs, said disaster official Bui Le Bac. In the low-lying areas, waters are reaching levels not seen since 1978.

The rains have now stopped, and it appears Vietnam may be spared Typhoon Megi, which slammed into the Philippines killing 20 and is headed to southern China. But waters here continue to rise. About 8 inches of rain drenched Ha Tinh province Tuesday, and officials discharged water from a reservoir hours later to protect the dam, worsening flooding in areas downstream and forcing the evacuation of thousands of people.

The bus Tuyen and Mung took was traveling from the Central Highlands province of Dak Nong to the northern province of Nam Dinh. It was caught on a section of Highway 1 that police were actually diverting traffic away from, as the Lam River overflowed its banks.

Like many Vietnamese, Tuyen could not swim, and Mung, a thin 46-year-old farmer, had not been in deep water herself since childhood. As they clung to the bus, she tried to prepare Tuyen for their ordeal, first telling him to remove his clothes and then to lie back and float. But he was too scared and there was no time. He decided to help others instead.

“He kept calling into the bus, urging the others to take the children outside and give them to the strong young men,” Mung said. “He managed to take out one child that was saved.”

Minutes later, the bus began to sink and Mung lost her hold on Tuyen. She was struggling to keep her own head above water, but she could still see her son.

“I saw him slowly disappear in the water and he yelled, ‘Mom, where are you?'” she recalled, gasping as she wiped her swollen eyes. “The current was so strong, I could not reach him. I still remember that image vividly of him slowly sinking with his hand waving, trying to ask for help.”

She then saw the bus roll onto its side and vanish into the murky water, with several screaming passengers still huddled inside.

Instead of giving up, Mung, a widow with two other children at home, channeled her grief into determination. She had to live so that someone could recover Tuyen’s body and give him a proper burial, allowing his spirit to rest.

She floated and thrashed in the swift-moving river for 2.5 miles, exhausting herself. At one point, the mother called out to her son, “I’m going to die with you because I can’t stand it anymore.”

But just when she was ready to stop kicking, she felt a hand reach for her, a fisherman — aged 19 like Tuyen — had spotted her and pulled her to safety.

Although she survived, she says she cannot live again until the river gives back her son. On Wednesday, she gathered with relatives near the site where the bus went down, praying and burning incense. As she tossed crackers, bottled water and fresh flowers into the river as offerings, she nearly collapsed while screaming her last wish into the swirling water.

“Tuyen, my son, come back to me! Do you hear what I’m telling you?” she cried, as a relative steadied her. “You must be very cold down there. Come back to me, I will not abandon you again.”

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Associated Press writer Tran Van Minh contributed to this report from Nghi Xuan.