Tumbling stocks just closed below a key technical threshold — and it could mean the pain is just beginning

The US stock market continued selling off in earnest on Monday, with all major indexes dropping more than 1.9%.
The S&P 500 closed below one of the market's most closely-watched technical levels, and historical precedent suggests the breach could lead to further selling.
It hasn't exactly been a smooth ride for US stocks in recent months. But no matter how turbulent conditions have gotten, market bulls have been able to find solace in the form of a key technical level on the S&P 500.
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