A Shiller CAPE ratio above 40 has only happened twice in 140 years of US market history.

The current Shiller CAPE ratio is higher than its peak before the 2008 financial crisis (around 27) and the 1929 crash (around 32).

The first time the Shiller CAPE ratio exceeded 40 was in December 1999, when it hit 44.19 right before the dot-com crash.