Almost $100 million in options tied to a barometer of U.S. stock market volatility changed hands in a split-second on Monday in what could be a hedge against falling stock prices.

About 40 different trades went off simultaneously at 12:16:04 p.m., encompassing just over 1 million contracts that would gain in value should the Chicago Board Options Exchange Volatility Index rise over the next few months, according to data compiled by Bloomberg.

The VIX tracks trading in options on the S&P 500 to indicate how investors expect the market to move over the next 30 days. Monday’s flurry of trades, Bloomberg suggested, may be a bet that the index will rise, which tends to happen when stocks fall.

“I’m not sure if it’s the biggest trade ever, but it’s certainly one of them,” said Jamie Tyrrell of Group One Trading LP, the primary market maker for VIX options. “Someone is interested in owning a lot of protection, but not in the near term.”

The four biggest of the 40 trades were each more than 130,000 contracts. While the timing of the trades was not proof they were connected, Bloomberg said, “it’s common on electronic venues for large blocks to be divided into smaller units.”

More than 1.5 million VIX options changed hands in total Monday, the highest daily volume since October, as the index rose 7.7% to 13.85. During the worst part of the 2008-2009 market collapse, the VIX soared to as high as 80.

According to Bloomberg, expectations of higher volatility have been creeping back into the market through options on the VIX. Traders owned about 5 million calls as of Friday, the most since November and more than double the open interest in January. They held 2.6 calls for every put, about the highest ratio since October.