The mass shooting in Las Vegas earlier this week brought the mood on Twitter to its lowest level ever Monday.

The Hedonometer, a tool that measures the happiness of millions of Twitter users based on their posts, showed an average happiness level of 5.77 after news of the massacre began spreading. The mass shooting claimed the lives of 59 people and left more than 500 injured when a gunman on the 32nd floor of a Las Vegas Strip casino started firing on concert-goers at an outdoor music festival below.

The previous record low on the Hedonometer was 5.84, recorded last year after another shooting killed 49 people at an Orlando, Florida nightclub. Twitter's third-saddest recorded day was Nov. 9, 2016, the day after Donald Trump was elected president of the US. The happiness level on that day was 5.87.

The tool, which has been tracking the mood of Twitter since 2008, was created by Peter Dodds and Chris Danforth, a mathematician and computer scientist, respectively, from the Advanced Computing Center at the University of Vermont. The tool measures mood through a system that assigns a value to words on a scale of one to nine based on their emotion. 

For example, "I feel extremely sad" garners a one, while "I am extremely happy" gets a nine.

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