When the Hindenburg exploded in 1937, killing 36 people, it made headlines around the world and earned a lasting place in history. Titanic’s sinking, causing the death of more than 1,500 people is an infamous event that can never be forgotten. Why is it, then, that when a dam failed and killed between 171,000 and 230,000 people, no one seems to have a clue that it ever took place? Read on to learn about the worst structural failure in history and how the Chinese government kept it a secret for 30 years.

The typhoon that hit the Henan province in eastern China on August 5, 1975, was one for the books. The region was accustomed to getting no more than 31.5 inches (80 cm) of rainfall in a year. This typhoon brought more than that each day. At its peak, the storm dumped 7.45 inches (18.95 cm) of water each hour, and 41.73 inches (106 cm) each day. The intensity of the rain was so great that birds covered the ground, pelted to death by the unrelenting water. The second day of the storm brought 16 hours of nonstop rain. The third day saw another 13 hours of precipitation.

The Ru River swelled to record levels, testing the limits of the Banqiao Dam. The dam was built in 1951 to control flooding and to provide electricity to the region. Built with the best that technology of the day offered, the dam was designed to handle an onslaught of rain as much as 1.65 feet (0.5 meters) of rainfall over three days. The typhoon brought six times that amount.

Workers at the dam were ordered not to release too much water, due to flooding further downstream. As the storm intensified, communication lines went down, leaving the isolated workers to guess what they should do.

On August 8, the river rose to near the top of the dam. In the late night hours, workers rushed frantically to pile sandbags on top of the dam, hoping to extend its protection. Shortly after midnight, the water level rose to one foot higher than the top of the dam. Just when everyone was beginning to despair, the rain stopped, the clouds parted, and exhausted workers looked up at the first sight of stars in several days. Sounds of rejoicing filled the night as they shouted, “The flood is retreating!”

Alas, the celebrations were premature. Within moments, the happy sounds of celebration were replaced with the horrifying cacophony. One witness described it, saying it “sounded like the sky was collapsing and the earth was cracking.” Another yelled, “The river dragon has come!”

The flooded area surrounding the failed Banqiao Dam.

The Banqiao Dam collapsed, sending, in one terrible moment, a massive wall of water nearly 20 feet (6.1 meters) high and over seven miles (11.27 kilometers) wide descended upon the villages below. Water equal to the contents of 300,000 Olympic swimming pools burst through the confines of the dam and set off a chain reaction of events. Before the night was over, the onslaught of uncontrolled water caused 62 dams to fail and drowning 26,000 people. In the days to come, the death toll climbed to between 171,000 and 230,000. The disaster destroyed about 5,960,000 buildings and impacted 11 million residents.

Most of the deaths in the days following the dam’s collapse were the result of starvation, exposure, or disease. Many people tried to survive by eating floating animal carcasses, but that only served to increase the spread of sickness.

Given the magnitude of the disaster, it is surprising that few know about it, even within China. The Maoist government immediately cloaked the disaster in secrecy. It wasn’t until 2005 that it was declassified and no longer considered an official state secret. As Eric Fish noted in the Economic Observer, the disaster “occurred in an era when the state quickly covered the scale of such catastrophes.” He also notes that even today it is rarely mentioned, since China is undergoing another round of rapid dam building.

Read more interesting facts about great disasters.

Read about the Boston Molasses Flood of 1919.

Read more fun facts about the weather.