The S&P 500 and Dow Jones Industrial Average turned green in a late surge after trading negative for most of the day, with a more than 800 point swing up by the Dow in less than two hours.

On top of Wednesday’s 5% surge, it was the biggest two-day rally in the S&P since August 2015.

“It’s hard to explain moves like today and yesterday and the last month,” said Sean O’Hara, president at Pacer ETFs. “From a time perspective, it’s a historic bull market and when you get that far into the cycle, people get more jittery.”

The S&P 500 has been careening toward its worst month of the record bull run and is down about 15% in the quarter as everything from higher interest rates to political turmoil in Washington to concern about global growth hammer at investor sentiment.

Havens came back in vogue, with Treasury 10-year yields slipping below 2.8%, and gold climbing with the yen.

The euphoria of equity investors evaporated earlier from Wednesday, when investors cheered a reminder of the American consumer’s strength and got reassurance on the tenure of the Federal Reserve chief and progress on US-China trade talks.

While there was no obvious catalyst for the return to selling that took stocks within a whisker of a bear market, the moves of the past few days sent volatility soaring.

Elsewhere, WTI crude oil prices gave up a slice of the more than 8% gain from the previous day. Losses in utility companies and carmakers dragged the Stoxx Europe 600 Index into the red.

Asian shares were mixed, though Tokyo’s Topix Index posted the biggest advance in two years.

“After a day like yesterday, you’d like to say, ‘Oh, that’s it. That kind of a move, that’s got to be the bottom.’ But you know what?” Michael Antonelli, equity sales trader at Robert W. Baird, said on Bloomberg TV. “We just don’t know yet.”