Today 09/28/21 the broad and deep decline in US stocks provided additional evidence that the next down wave could be underway.
The daily S&P 500 (SPX) courtesy of Trading View illustrates the action.

The daily RSI + MA had a bearish crossover following the daily Stochastic bearish crossover on 09/27/21.
A Fibonacci extension from the 09/23/21 SPX high at 4465.40 targets SPX 4077.18 as the next possible support zone. Note its in the same general area of the May 2021 minor correction bottoms.
I suspect that if within the next few weeks, the SPX continues to decline it could break below its 200 – day Moving Average (MA). If the SPX can break below the 200 – day MA, it could be the most important clue of a developing bear market.