The U.S. is approaching the end of a "superbubble" spanning across stocks, bonds, real estate and commodities following massive stimulus during the COVID pandemic, potentially leading to the largest markdown of wealth in its history once pessimism returns to rule markets, according to legendary investor Jeremy Grantham. 

"For the first time in the U.S. we have simultaneous bubbles across all major asset classes," said Grantham, co-founder of investment firm GMO, in a paper Thursday. He estimated wealth losses could total $35 trillion in the U.S. should valuations across major asset classes return two-thirds of the way to historical norms.

"One of the main reasons I deplore superbubbles — and resent the Fed and other financial authorities for allowing and facilitating them — is the underrecognized damage that bubbles cause as they deflate," said Grantham.