This is the Mullaperiyar Dam. It has been around since well before your grandparents were born. And it could fail at any moment now. What could cause the dam to collapse? How quickly would this lead to a disaster? And in what ways could this devastate the lives of people living close by?

The Mullaperiyar Dam is located in India’s Kerala state. For over 100 years, the dam has provided vital power and irrigation to Kerala and neighboring Tamil Nadu. But the days could be numbered for this 54 m (177 ft) high dam. Its design and technology are not as advanced as current standards require.

To make matters worse, the region is prone to earthquakes. A lot of them. In 1979 and 2011, earthquakes left structural cracks in the dam. Could the next quake crack the dam wide open, threatening the lives of millions of people?

The day would start quietly and peacefully. You’d be standing on the Mullaperiyar Dam admiring its effectiveness after so many years of service. On one side, there’s a controlled, powerful rush of water. On the other side is a massive reservoir. This dam has a volume of 443 million cubic m (579 million cubic yd). And in the monsoon season, it would be full to the brim.

Water would barely top over the dam’s wall. But your morning visit would get shaken up as the ground suddenly trembles below you. That was a minor earthquake. It would only last a few seconds and then life would return to normal, or so you’d think.

You’d hear a loud cracking noise and run to check the dam’s edge. The earthquake expanded a small crack in the dam’s bank. And now, water would start flowing through it. A little at first, then more and more. Kerala, we’ve got a problem. You’d realize pretty quickly that the hole reached all the way to the reservoir.

Luckily your survival instincts would kick in, and you’d run to the side and get off the dam. You’d just make it to safety before the hole gives way to a catastrophic breach. Now the real trouble would begin. A huge mass of dirty water would burst through the structure. It would take nearly everything in its path along with it.

People nearby would face the danger of an uncontrolled, rushing river of soil and rock debris with enough water to fill 180,000 Olympic-sized swimming pools. You’d count your lucky stars that you so narrowly escaped death. But now, from your vantage point at the dam’s side, you’d watch all the destruction happening.

You’d see people trying to run away, only to be twisted up and mangled in the flowing water. Anyone sheltering inside a building wouldn’t be safe either. The flood would rip whole buildings off their foundations. In the distance, roads and bridges would get washed away too. The lives and livelihoods of about 100,000 people would be devastated before they could get to safety.

But the dangers would not be over yet. After about 45 minutes, the flood would reach the Idukki dam about 25 km (15 mi) downstream. Theoretically, this dam should be able to withstand the Mullaperiyar’s floodwater. But its full reservoir could doom the Idukki dam. And if all the pressure would cause the Idukki Dam to breach. All 15 interconnected dams leading to the Arabian sea could fail too.

That would directly impact the lives of over 3.5 million people, leading to a grim future for you and the rest of the survivors. Everywhere you go, you’d be trudging through debris-filled mud. And the water all around you would likely make you sick. That’s because the flooding would contaminate the water with a huge variety of metals, chemicals and wastes.

You and all the other survivors would probably need to relocate to other places. Beyond the human cost of this tragedy, the dam would need to be redesigned and rebuilt. The cost would be in the billions. The sad reality is that this scenario is not a matter of if. It’s a matter of when. The regional governments that rely on the dam have been fighting each other in court about the dam’s fate.

And recent report from the UN University’s Water, Environment and Health institute recommends the dam be decommissioned. If this scenario happened, the whole world would witness another tragedy that could have been avoided.

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