The 's seventh-straight loss was especially worrisome to traders who follow charts because the benchmark closed below 2,100.
Many of them predict deeper losses ahead if the index can't bounce quickly back above this level before the week closes out.
S&P 500 3 years:
This level became key to the market after the S&P 500 first passed it in early 2015 and then struggled to break meaningfully above it for almost the next year and a half. After the benchmark rallied clearly above 2,100 in July, this key level of resistance then became key support.
CNBC PRO asked Wall Street technical analysts for their current take on the prospects for the market and the next important support levels.
Here is what they see.