The latest horrifying video of an out-of-control escalator circulated on social media this week, possibly the result of too many people on an escalator that critics of Rome’s infrastructure say is a sign of the city’s overall disrepair.

Fans of Russia’s CSKA Moscow soccer team, in the Italian capital after a match with AS Roma in the Champions League on Tuesday, can be seen heading down an escalator at the Repubblica Metro Station before the escalator fails. The New York Times reports that the Russian Embassy accounted for 16 Russians who were injured, with three in serious condition.

“The scene that we found was people piled up at the bottom of the escalator,” said Rome provincial fire chief Giampietro Boscaino, per The Washington Post. “People one on the top of the other, looking for help. They had various injuries caused by the escalator that was twisted, therefore serious injuries.”

Rome’s transportation agency, ATAC, has opened an investigation into why the escalator failed. The Guardian reported this week comments from a source that the failure was likely from “maintenance negligence.” The source told the UK newspaper that a steel cable might be to blame. The cable that prevents the escalator steps from turning into a terror slide might have failed. The escalator was reportedly only ten years old.

So, why do escalators break like we saw this week, seemingly losing all control?

Think of an escalator like a large belt that rotates with a motor. When escalators are operating nominally, the motor keeps the cascading staircase moving at an even speed, as the GIF below shows:

This GIF shows essentially how escalators work. Under too much weight or force, they can break.

But too much weight pressing down on the steps can put stress on a cable that keeps the steps upright, as each person standing on an escalator step adds slightly their own weight and force. Too much force downward and forward can wreak havoc.

A similar incident appears to have happened in this video, posted to Youtube in 2013:

The result is traumatic, and can result in injuries, like the ones we see the fire brigade in Rome addressing in this video below:

The caption in this tweet from Italy's national fire department in part reads, "all the injured have been rescued and sent to hospitals. The safety intervention continues."

The photo below also shows how the steps that moved so quickly were pushed off their track and onto the surface instead of folding back underneath for another rotation:

The cpation from Italy's national fire department in part reads, "all the injured have been rescued and sent to hospitals. The safety intervention continues."
This photo from Italy's national fire department shows the foot of the escalator that failed.

Rome’s crumbling infrastructure has been the source of civic protests in recent years, and on Saturday, a political group called the Five Star Movement will protest the decline.

Photos via Vigil del Fuoco